Along with natural disasters, infectious diseases are among the top unintentional causes of human death and suffering the world over. Some diseases have left their mark on the human race, warping the course of human history in their wake.

In certain cases, like that of the bubonic plague, population levels were drastically reduced for centuries afterward. In other cases, such as polio, the infection of a renowned individual led to the further recognition of a disease and the need for a cure.

10 Bubonic Plague

The bubonic plague (aka the “Black Death“) spread across Europe from east to west during the 14th century. The Yersinia pestis bacterium was responsible for the epidemic and used Oriental rat fleas as its intermediary in reaching the human population. Rats, which carried infected fleas, traveled west on the Silk Road and on ships across the Mediterranean.

The bubonic plague demonstrated early on how human advancements in commerce and trade could fatally spread a pathogen. The plague’s name comes from the Latin word bubo, referring to a pustule or abscess.

The symptoms were gruesome. They started with fever and sweating but progressed to blackish-blue boils across the groin. If the boils weren’t lanced, they grew and people would die from the toxic buildup. Likewise, lancing the buboes was often just as deadly and could lead to the pathogen becoming airborne.

The mortality rate with this disease was upward of 70 percent, killing up to 200 million across Europe and cutting the continent’s population in half.^[1] Historians believe that the spread of the bubonic plague contributed to the fall of the feudal economic system and caused irreparable damage to the church.

Many priests were infected after performing last rites and funeral services. Still more withdrew from their parishes, afraid of contracting the plague. To this day, the bubonic plague ranks among history’s most gruesome diseases even as the development of antibiotics has limited the Black Death’s contemporary occurrences.

9 Smallpox

When Europeans first arrived in the New World during the late 15th and early 16th centuries, they used advanced military techniques to conquer North and South America with haste. But they also brought smallpox, which played an instrumental role in killing Native Americans.

Europeans from the Old World had a long history of living in close quarters with domesticated animals as well as eating and drinking from similar sources. This led to the spread of many diseases. But those who survived developed an impressive immunity to otherwise deadly pathogens. These individuals were among America’s first settlers, who brought smallpox to the continents as early as 1520.

In conjunction with other Old World diseases like the flu and measles, smallpox went on to kill almost 90 percent of the Native American population, far outpacing the damage done by late medieval warfare. Smallpox was also a vicious deforming agent, leaving those infected with noticeable sores across their bodies.^[2]

Fast-forward several centuries, and smallpox is one of just two diseases (the other is rinderpest) to be fully eradicated from the human population due to vaccination efforts. Today, smallpox can only be found in exceedingly guarded laboratory settings.

8 Spanish Influenza

The 1918 flu pandemic was caused by one of the deadliest 20th-century pathogens, infecting 500 million individuals worldwide. Outbreaks in the United States and Europe soon spread around the world.

Although this deadly strain of the flu ravaged population centers indiscriminately, it quickly gained the moniker “Spanish influenza” as Spain was hit particularly hard by the bug. Even Spain’s King Alfonso XIII contracted it.

SSpanish influenza had a noticeable effect on World War I’s battlefields as it infected many young, otherwise healthy individuals. Records from the time suggest that more Americans were killed by the 1918 flu than from fighting on the front lines.

Forty percent of servicemen in the US Navy contracted the flu. Almost as many in the US Army were afflicted as well. The flu’s effects were noted across the economy and military, with many believing that the outbreak influenced the direction of World War I by infecting militias and destroying medical infrastructure.^[3]

Flu vaccines were first developed in the 1940s.

7 Polio

Today, polio is exceedingly rare, with relatively few cases in the industrialized world since Jonas Salk developed a polio vaccine. Prior to the creation of a vaccine, polio was easily transmitted in an infected individual’s stool or via droplets when he sneezed.

Polio is commonly asymptomatic. Yet when symptoms do present themselves, they can be debilitating. The disease is notorious for paralyzing its victims, requiring them to live the rest of their lives in iron lungs. Paralysis caused by polio can’t be reversed. Although mobile generators can aid some of the afflicted, others still rely on the iron lungs made famous in the 1940s.^[4]

The best-known individual to suffer polio paralysis was former US President Franklin D. Roosevelt. His efforts to combat the stigma of the disease touched every aspect of the presidency in the years leading up to and through World War II. Ultimately, this changed how a man—and a nation—viewed paralysis-inducing disease and subsequent disability.

6 Syphilis

There are four stages of syphilis, a sexually transmitted disease that first appears with a benign chancre at the spot of infection. Secondary syphilis presents with a widespread rash and swollen lymph nodes. The bacteria then enters a latent stage before surfacing as tertiary syphilis, which leads to neuromuscular degeneration, blindness, and dementia.

Historians are unsure as to how syphilis got a footing in European populations, but the leading hypothesis says that it was an import from New World colonization. Syphilis has marred many famous individuals, including several members of the medieval papacy. In 1508, Julius II was unable to have his foot kissed by fellow Christians because it was covered in syphilitic sores.^[5]

Syphilis also had a long history of inspiring worthless medical remedies such as the use of mercury, which often left the infected worse off. Today, syphilis is still widespread, though it can often be cured with penicillin.

5 HIV/AIDS

Few diseases have carried the stigma of human immunodeficiency virus (HIV), which transforms into acquired immunodeficiency syndrome (AIDS). Scientists believe that the virus crossed over from primates to humans in Africa during the early 20th century. Yet the disease didn’t gain traction in popular culture until the early 1980s when several gay men in New York and California exhibited strange cases of pneumonia and cancer.^[6]

Its initial association with gay men led to the early name “gay-related immune deficiency” (GRID). Paranoia swept Europe and the United States as individuals were unsure what modes were responsible for spreading the disease. HIV’s association with the gay community led to the development of activist groups like ACT UP, helping to propel early LGBT advocacy and solidify eventual rights for sexual minorities decades later.

4 Tuberculosis

Tuberculosis (TB) is a deadly respiratory infection that can take two forms: latent TB and active TB. Latent TB isn’t contagious, and one’s immune system can often fight it off. In fact, one-third of the world’s population has latent TB.

In weakened immune systems, active TB can take hold. Symptoms include bouts of coughing, severe chest pain, night sweats, and loss of appetite. The rise of HIV/AIDS is linked to a rise in TB cases as weakened immune systems have a nearly impossible time fighting off the otherwise latent bacteria.

Tuberculosis left its mark on science in more than one way. In the 19th century, TB was often spread through milk. This led to the development of batch pasteurization, a low-temperature method of pasteurization which has its roots in eradicating tuberculosis in dairy products.^[7]

3 Malaria

Malaria is a mosquito-borne disease caused by parasites that leave those infected with flu-like symptoms. Malaria remains one of the world’s most serious killers, infecting more than 200 million in 2016 and killing almost 500,000.

Malaria’s common claim to fame is its possible role in killing Alexander the Great. But did you know that malaria—and resistance to it—lent a hand in the brutal transatlantic slave trade? There is no evidence of malaria in precolonial America, and thus its introduction ravaged native populations.

This led early European populations to look to the people of Africa, who were resistant to malaria if they survived the disease as children, to work on America’s Southern plantations. As malaria spread like wildfire across the American South, killing off Native Americans, Africans were forced into the slave trade.^[8]

On the medical side, malaria provided scientists with a fundamental understanding of transmission vectors, including how disease-carrying animals can infect human populations and what can be done to stop them.

2 Ebola

Few diseases have fueled hysteria quite like Ebola, which was only discovered in Africa in the late 1970s. Ebola, short for Ebola hemorrhagic fever (EHF), is a virus that leads to extreme bleeding in humans and other primates. Symptoms can take several days to weeks to develop. They include sore throat, muscular pain, vomiting, diarrhea, and eventual internal and external bleeding.

Depending on the strain, Ebola comes with a high mortality rate, killing almost half of those it infects. However, mortality rates can run as high as 90 percent. The deadliest Ebola outbreak spread out of West Africa in March 2014. It killed five times more individuals than all previous outbreaks combined.

Cases were reported in the United States and Europe (including the United Kingdom, France, Germany, and Spain). Full containment of the virus didn’t occur until 2016. Its spread and aftermath dramatically tested the World Health Organization’s ability to respond to a modern pandemic.^[9]

1 Cholera

At its worst, cholera can go from asymptomatic to fatal in just under three hours. Cholera is a diarrheal disease caused by a bacterium that is usually spread through water or food systems that lack proper sanitation. Although the roots of the disease were in the Ganges Delta in India, cholera has since spread across the planet. There have been pandemics in South Asia (1961), Africa (1971), and the Americas (1991).

Every year, there are as many as four million cases of cholera with more than 100,000 dying as a result.^[10] As of July 28, 2010, the United Nations resolved to explicitly recognize clean drinking water as a human right, a development inherently tied to the spread of water-borne bacteria.

Evan Beck is a freelance writer living in the San Francisco Bay area.

The Spanish flu often gets overshadowed by its more popular counterparts like Ebola and the Black Plague. In many ways, though, it was far deadlier than anything we’ve seen before or since. An estimated one-third of the entire world’s population contracted it, killing at least 50 million people. The doctors of 1918 didn’t even know what it was, as they didn’t quite understand viruses and influenza pandemics back then.

Despite being such a world-changing event, the Spanish flu is surprisingly underrepresented in popular world history. While we understand that other important things—like the Russian Revolution and World War I—were also going on around that time, they still don’t compare to what many historians have described as the “greatest medical holocaust in human history.”^[1]

10 The Spanish Connection

Because it’s called the “Spanish flu,” many of us assume that it originated in Spain. Its name is the primary reason we still blame Spain for something it had nothing to do with. Even though we don’t know where the flu came from, we know for sure that it wasn’t Spain. In fact, the number of cases in Spain was lower than in many other countries.

It’s called Spanish flu because Spain was one of the first (maybe the first) countries to widely report on it. Because of being neutral in World War I (they kind of had their own thing going), their press was freer than other nations’. Countries like Britain, the US, and Russia, all of which were involved in the war, kept the news of the flu restricted to maintain morale. It’s a classic case of blaming the messenger, as people started attributing the flu to Spain, even if it was just honestly reporting on it.^[2]

9 We Kind oOf Ignored It at the Time

As already mentioned, the Spanish flu was a pandemic unlike any other we had ever seen in our history, and it spread to almost every part of the world in a matter of months. For a medical scare as big as that, you’d think that it would make the front pages of all the leading newspapers of the time.

In reality, though, all we cared about back then was the Great War. At that time, it was the biggest and only global conflict people had seen. It was a catchier way to die than the flu, as “people dead due to disease” wasn’t exactly breaking news back in 1918, but “people dead due to chlorine gas in battle” absolutely was.^[3]

8 It Wasn’t a Super Strain of the Flu

The sheer speed of infection and number of deaths in such a short period of time really doesn’t sound like something a normal flu virus can do. The Spanish flu is always assumed to be some super strain that we just didn’t have a cure for, which came out of nowhere and decimated entire swaths of our population.

Recent research, however, has found that the virus wasn’t more aggressive or dangerous than any previous influenza epidemics.^[4] The primary reasons it was so deadly were overcrowding and poor hygiene due to the war. Many people around the world found themselves in cramped spaces like military barracks and refugee camps because of geopolitical conditions at that time, which allowed the virus to spread around far more easily than it would have otherwise.

7 Resurrection of the Virus

The Spanish flu has been a subject of study ever since it first struck, mostly due to how fast and deadly it was. Despite all of our efforts trying to understand it, though, for the longest time, we had no idea about what caused it or what kind of virus it even was. That was until the 2000s when some scientists isolated it and figured that recreating those conditions in the lab was the best course of action.

In a move that could have gone very, very wrong for humanity, some researchers got samples from frozen bodies of the infected and sort of activated it in the lab to see what it does. They were surprised to find out that the virus was similar to the H5N1 bird flu virus, which had been responsible for some deaths in Asia the previous year.^[5]

It’s concerning news. Bird flus, in general, are turning more pathogenic with every subsequent outbreak, and if any ever become really good at infecting humans, the resulting pandemic could be worse than the Spanish flu.

6 The Flu Virus Wasn’t Actually the Main Killer

One of the biggest concerns regarding the Spanish flu is that we don’t really know how to battle it, even with our modern medical technology. For one, various pathogens today are rapidly developing immunity against antibiotics and other medicines, largely owing to our overwhelming reliance on such medicines to cure even simple ailments.

More importantly, influenza viruses in themselves aren’t the problem; it’s the other medical complications that come with them that we should really be worried about. Studies have found that the majority of deaths during the Spanish flu were actually due to bacterial pneumonia, which took hold immediately after the virus. Once the flu had successfully weakened the immune systems of those affected, their bodies were fertile grounds for the bacteria.^[6]

5 Why It Affected Healthy Young Adults

One very peculiar aspect of the whole thing was the fact that the Spanish flu mostly affected healthy, young adults, which was also the demographic most actively participating in the war. (It’s probably not a coincidence.) It was one of the most perplexing parts of it all, as it flew against everything we thought we knew about flu outbreaks or disease in general. Young adults should be much better equipped to fight off disease than other groups, like the elderly, though that clearly wasn’t the case this time. So, what gives?

According to recent research, it has a perfectly plausible explanation. People born between 1880 and 1900—the most-affected demographic—never developed immunity toward the right type of flu viruses. The flu that was most prominent during their childhoods was distinctly different from the Spanish flu. Those born earlier in the 19th century had been exposed to flu viruses more like the Spanish flu and thus had better immunity.^[7]

4 Aspirin Might Have Made It Worse

There’s no doubt that the Spanish flu was one of the deadliest events in human history. It was a humbling experience, and no one could have stopped it, though it’d be a bit unfair to say that we weren’t at fault at all.

According to some scientists, many of the early deaths when the flu broke out may have been due to aspirin. You see, back then, we had little idea of what a normal dosage of aspirin should be. We were using it like water, just giving it to patients in the hope that they’d get better, as we didn’t have any better ideas. The scientists hypothesized that “a significant proportion” of those deaths happened due to salicylate poisoning, as we hadn’t yet figured out the upper limit to how much aspirin people can safely consume.^[8]

3 Corpse Exhibitions

One part that’s left out of all the Spanish flu stories is the horror of it all. Imagine going out to buy groceries and seeing rotting corpses and severely sick people on the side of the road before you even make it to the store. Of course, that’s if you were lucky enough not to get infected yourself, as the chances of that were quite high, too.

One particularly horrifying aspect was what to do with all the dead. The number of deaths in the U.S. was so high that it wasn’t always possible to build coffins for everyone, so the dead were directly buried in the ground. This was referred to as “planting.”

If that wasn’t bad enough, many buildings also served as temporary morgues to store the dead. In one particularly creepy case, bodies were displayed in the windows of the high school in West Pittston, Pennsylvania, so that loved ones could pay their respects from a safe distance.^[9] (The school had already been closed to prevent the spread of the disease.)

2 It May Have Originated in China

Apart from being an unprecedented medical emergency, the Spanish flu was also quite mysterious. The biggest question was where it came from in the first place. (We’ve already established that it wasn’t Spain.) Knowing where it came from is crucial to identifying the conditions that led to such a severe outbreak and making sure it doesn’t happen again. While this is a question we may never be able to decisively answer due to a lack of medical records from the time, some historians think the Spanish flu may have come from China.

According to Mark Humphries from the Memorial University of Newfoundland, the virus was carried to Europe with some 96,000 Chinese laborers hired to work with the British and French forces. While he admits that we’d need more samples of the virus to truly confirm its origins, other historians find this hypothesis plausible. China had a lower number of casualties from the flu, which suggests that their population had prior immunity to it.^[10]

1 One Of The Biggest Threats To Human Life

When we think about threats to life on Earth, we always think of stuff like nuclear war or asteroids. That’s probably because those events sound a lot more destructive and deadly than a mere disease, not to mention the medical care of today being a lot more evolved than before.

Unfortunately, as recent outbreaks like H5N1 and Ebola have shown, a rapidly spreading pandemic is still on top of the list of things with the ability to wipe out human civilization. Sure, our quarantine methods and medicines are quite capable of dealing with such threats, though that’s only because we haven’t yet encountered anything like the Spanish flu since 1918. The same flu today would spread much faster due to the world being much more connected than back then. The virus could also be much more resistant to our medicines.

By some estimates, a virus with comparable pathogenicity to the Spanish flu would kill more than 100 million people today, and there’s little we’d be able to do to contain it.^[11] We’re not saying that we should all start panicking, but world governments would do well to focus a bit more on this threat.

Yet, eerily, no one really listened as this post was published mere months before the world was hit with another global pandemic—SARS-CoV-2 or COVID-19!

You can check out Himanshu’s stuff at Cracked and Screen Rant, or get in touch with him for writing gigs.

Amid a global outbreak, it is easy to forget that humanity has triumphed over many diseases that used to devastate entire countries. Today, vaccinations are estimated to save three million people from death every year across the globe.

A few diseases have even been eradicated (gone forever), with even more eliminated (gone from entire countries or regions). Although we have a long way to go, it is comforting to look back on where we’ve been. Here are 10 of the greatest triumphs of modern science over bacteria, viruses, and parasites.

10 Infectious Diseases That Changed History

10 Smallpox

Smallpox was an airborne virus that killed roughly one-third of the people who contracted it. There is no cure, so doctors simply waited about two weeks to see if the patients died. If they were lucky enough to survive, they were permanently scarred from the iconic red pustules that give the disease its name.

In 1796, a British physician named Edward Jenner noticed that people who had cowpox when they were younger never seemed to catch smallpox. To test this theory, he injected the eight-year-old son of his gardener with cowpox before exposing the kid to the deadly smallpox virus. Surprisingly, it worked. Jenner had stumbled upon the world’s first vaccine.^[1]

His discovery significantly reduced the number of deaths from the disease. By the time the World Health Organization (WHO) announced its plan to eradicate smallpox in 1959, the disease was restricted to South America, Africa, and Southeast Asia.

WHO ordered massive amounts of vaccines and instructed local governments on how to administer them. In 1977, the final recorded case of smallpox was seen in Somalia. As a result, WHO officially announced in 1980 that smallpox was the first disease ever to be eradicated by human intervention.

9 Rinderpest

Rinderpest (aka “cattle plague”) did not infect humans but was still devastating. In the 1890s, an outbreak killed 80–90 percent of all cattle in sub-Saharan Africa. Those who depended on these animals for meat, milk, and farmwork starved to death. The population of affected areas was reduced by at least one-third.

Any animal that survived the disease would never get rinderpest again. Also, the high death rate meant that wild animals (such as buffalo or giraffes) with rinderpest were wiped out before they could pass the virus to domestic cattle.

In the 1960s, a British scientist named Walter Plowright developed a vaccine, which the United Nations Food and Agriculture Organization started buying in bulk in the 1990s. Thousands of veterinarians across Europe, Asia, and Africa participated in vaccinating cattle and finally succeeded in wiping out rinderpest in 2011. It is the second disease to be eradicated.^[2]

8 Polio

Polio, short for poliomyelitis, is a virus that paralyzes its victims. Sometimes, the paralysis is temporary. Other times, it causes permanent disability. Worst-case scenario, it spreads to the lungs and the patient dies.

In 1953, Jonas Salk announced that he had developed a vaccine for polio. He refused to patent his discovery in order to get it to as many people as possible. As a result, he forfeited billions in today’s US dollars.

The March of Dimes funded a large-scale trial of the vaccine before it was mass-produced for the US public. In 1979, only 24 years after the vaccination was formally introduced, the United States was declared polio-free.

To eliminate polio worldwide, WHO created the Global Polio Eradication Initiative in 1988. Between 1980 and 2016, there was a 99.99 percent decrease in cases. Currently, polio is only seen in Afghanistan, Pakistan, and Nigeria.^[3]

7 Guinea Worm

When a person drinks stagnant water contaminated with Guinea worm larvae, the worms travel down to the intestines to mate. After 10–14 months, the female worm migrates as far as the human foot. There, she creates a painful skin lesion that drives the host to soak his foot in a pond or river, which releases the larvae into the water. From there, the cycle repeats.

Despite the painful lesions and the potential for bacterial infection, Guinea worm is rarely deadly. But as a general rule, people prefer not to have worms reproducing in their intestines.^[4]

The Carter Center is leading the charge to use water purification to prevent the spread of Guinea worm. So far, it’s been incredibly successful. McGill University estimates that there were 3.5 million cases in 1986 throughout South Asia, Yemen, and sub-Saharan Africa. In 2018, there were 28 reported cases worldwide.

6 Yaws

Yaws (aka frambesia) is caused by skin-to-skin contact with the bacterium Treponema pallidum pertenue. Within three months of being infected, the patient will develop raspberry-like lesions on his skin, especially his face. These vanish within six months, only to erupt again later and leave scarring. Humans being humans, these scars can lead to bullying and discrimination.

In the 1950s, yaws was endemic in over 70 countries. Since it is easily cured with a single dose of the antibiotic azithromycin, it was one of the first diseases targeted by WHO for eradication. Yaws was almost wiped out in the 1960s, but smallpox received more attention and interest fell in yaws. In 2019, yaws was still present in 15 countries.^[5]

10 Diseases That Prevent Other Diseases

5 Hookworm

Hookworms, which live in sewage-contaminated soil and enter through the patient’s feet, travel up to the small intestine where they drink their victim’s blood. From there, they cause lethargy, anemia, bloating, and stunted growth.

As they live in areas with poor sewage treatment, the patient often poops them out and the hookworms end up in the ground again. From there, they find new feet to invade.^[6]

In 1910, John D. Rockefeller Sr. donated $1 million to eradicate hookworms in the American South. After a five-year campaign, infection rates were significantly reduced. Even better, more kids were showing up to school and actually paying attention because they didn’t have worms sapping their energy.

Despite the near elimination of hookworms, there is a high rate of reinfection and the worms can reappear. To conquer hookworms once and for all, the Human Hookworm Vaccine Initiative is creating a drug that interrupts the worm’s ability to eat. Soon, they might be gone for good.

4 Measles

Yes, this one has made a bit of a comeback in recent years because people haven’t continued to vaccinate against it. But globally, measles cases have been drastically reduced in the past 50 years.

Before 1963, just about every child in the United States got measles before age 15. Major epidemics occurred every 2–3 years, with an average of 2.6 million deaths every year due to the disease. It is passed through coughing and sneezing.

In 1954, Dr. Thomas C. Peebles of Harvard University asked sick, 11-year-old David Edmonston if he wanted “to be of service to mankind.” David let the doctor swab his measles-infected throat. Physician John F. Enders was able to isolate the virus from his sample and created the Edmonston-Enders vaccine that is still used today.^[7]

Between 2000 and 2018, measles deaths dropped 73 percent thanks to mass vaccination, saving an estimated 23.2 million people. Even in the United States (where measles was eliminated in 2000), only 1,282 cases appeared in 2019.

3 Tetanus

Tetanus is caused by the bacterium Clostridium tetani, which lives in soils all over the world. The bacteria enter the body through an open wound, where they release a toxic chemical that causes paralysis and painful muscle contractions.

As C. tetani is resistant to chemicals and heat, tetanus is unlikely to be eradicated. But it can be eliminated (no new cases) through mass vaccination. So far, efforts have been incredibly successful.

In 1990, 314,000 people died of tetanus worldwide. In 2017, there were only 38,000 deaths. That’s an 88 percent decrease. The highest numbers of cases are in South Sudan and Somalia.^[8]

2 Elephantiasis

Elephantiasis is caused by three species of threadlike worms that lodge in their victim’s lymphatic system. This disrupts the body’s ability to regulate liquids, which can cause the infected limbs or genitals to swell to elephant-like proportions.

While the worms are in the lymph nodes, they release larvae into the bloodstream. So, if a mosquito bites someone with elephantiasis, it passes the disease to the other people it bites.

Certain medications can be used to treat the illness. Combined with precautions against mosquitoes, this can stop the spread of the disease.

Since 2000, at least 7.7 billion treatments have been delivered to more than 910 million people. Sixteen countries have eliminated elephantiasis, and seven more are under surveillance to see if they can make the list.^[9]

1 Malaria

Malaria is caused by a small parasite in the blood that is spread by mosquitoes. Symptoms include fever, chills, headache, nausea, and body aches. Although the disease continues to infect people across the world, humanity has made enormous leaps in combating it.

Malaria has been endemic in Europe, Africa, and Asia since time immemorial. When Europeans colonized North and South America, the disease hitched a ride. Historians estimate that, at the height of the disease, inhabitants across 53 percent of the world’s landmass faced the risk of contracting malaria.

In the United States and Europe, the 1900s brought new understanding as to how malaria spread. Wetlands, where mosquitoes bred, were drained or filled in. Insecticides were mass sprayed. One organization, the Office of Malaria Control in War Areas, later became the Centers for Disease Control.

The 21st century has been incredibly productive in the fight against malaria. Between 2000 and 2015, the number of malaria deaths dropped from 840,000 a year to 440,000. Most of those who died were children in Africa.^[10]

10 Surprising Facts About The Spread Of Disease