Alongside earthquakes, tsunamis and other natural catastrophes, infectious diseases rank among the deadliest unintentional killers humanity has ever faced. The 10 infectious diseases listed below each left an indelible imprint on the course of civilization, redirecting empires, economies and everyday life.
Why 10 Infectious Diseases Matter
10 Bubonic Plague
The Black Death, formally known as the bubonic plague, swept westward across medieval Europe in the 14th century, carried by the bacterium Yersinia pestis and the Oriental rat flea that hopped aboard Silk Road caravans and Mediterranean vessels.
This catastrophe was an early warning that the very advances in trade and commerce that enriched societies could also serve as a lightning‑rod for deadly microbes. The disease’s name stems from the Latin “bubo,” meaning a swollen pustule or abscess.
Symptoms were terrifyingly graphic: an initial fever and sweats gave way to black‑blue buboes that festered in the groin. If left untouched, they swelled and released toxins; paradoxically, attempts to lance them often turned the infection airborne.
Mortality ran sky‑high—over 70 % of those infected died, amounting to as many as 200 million souls and halving Europe’s population. Historians link the plague to the collapse of feudalism and a crippling blow to the medieval Church.
Clergy were hit hard, with many priests succumbing after performing last rites, while others abandoned their parishes out of fear. Modern antibiotics have tamed the disease, but the Black Death endures as one of history’s most gruesome scourges.
9 Smallpox
When European conquistadors crossed the Atlantic in the late 1400s and early 1500s, they brought more than swords and steel—they also carried smallpox, a virus that would decimate indigenous populations across the New World.
Old‑World societies lived in close proximity to domesticated animals and shared water and food sources, fostering a suite of ailments. Those who survived built robust immunity, and these hardy settlers inadvertently introduced smallpox to the Americas as early as 1520.
Combined with other imported illnesses like influenza and measles, smallpox annihilated roughly 90 % of Native Americans, eclipsing the death toll of contemporary warfare. The disease also left disfiguring pustules on survivors’ skin.
Centuries later, smallpox became one of only two diseases ever eradicated worldwide—the other being rinderpest—thanks to aggressive vaccination campaigns. Today, the virus lives only in highly secured laboratories.
8 Spanish Influenza
The 1918 influenza pandemic, often called the Spanish flu, infected an estimated 500 million people worldwide, making it one of the deadliest 20th‑century pathogens.
Although the virus struck indiscriminately, it earned the moniker “Spanish” because Spain, a neutral country during World War I, reported the outbreak more openly than warring nations.
Spanish influenza dramatically affected the Great War’s battlefields, striking down countless healthy young soldiers. In the United States, more citizens died from the flu than from combat.
Forty percent of U.S. Navy personnel contracted the illness, with a similar rate among Army troops. The pandemic’s reach extended to economies and medical infrastructures, prompting speculation that it altered the war’s trajectory.
It wasn’t until the 1940s that scientists finally produced a flu vaccine, offering a shield against future pandemics.
7 Polio
Before the development of Jonas Salk’s vaccine, polio roamed unchecked, spreading through contaminated stool or respiratory droplets when an infected individual sneezed.
Most infections are silent, but when symptoms emerge they can be devastating. The disease is notorious for causing irreversible paralysis, often confining victims to iron lungs—those massive, noisy machines that kept patients breathing.
The most famous polio survivor was President Franklin D. Roosevelt, whose public battle with the disease reshaped attitudes toward disability and spurred nationwide efforts to combat the illness.
6 Syphilis
Syphilis progresses through four distinct stages: an initial painless chancre, a secondary rash with swollen lymph nodes, a latent period, and finally tertiary disease that can devastate the nervous system, eyes and brain.
Scholars still debate how the bacterium arrived in Europe, but the prevailing theory points to the Columbian exchange after the New World’s colonization. The disease even plagued members of the medieval papacy; in 1508, Pope Julius II was forced to keep his foot away from the faithful because it was riddled with syphilitic sores.
Historically, physicians prescribed toxic mercury treatments that often worsened patients’ conditions. Modern medicine now cures most cases with a simple course of penicillin.
5 HIV/AIDS
The human immunodeficiency virus (HIV), which can evolve into acquired immunodeficiency syndrome (AIDS), carries a heavy social stigma. Scientists think the virus leapt from African primates to humans in the early 20th century, but it didn’t enter public consciousness until the early 1980s, when clusters of rare cancers and pneumonia appeared among gay men in New York and California.
Initially labeled “gay‑related immune deficiency” (GRID), the disease sparked widespread fear and misinformation. Activist groups such as ACT UP emerged, using the crisis to galvanize LGBTQ rights and push for better research, treatment and public policy.
4 Tuberculosis
Tuberculosis (TB) is a deadly respiratory infection that can exist in a silent, latent form or erupt into an active, contagious disease. Roughly one‑third of the world’s population carries latent TB without symptoms.
When a person’s immune system is compromised—such as by HIV/AIDS—TB can become active, causing relentless coughing, chest pain, night sweats and loss of appetite.
The fight against TB spurred scientific breakthroughs; in the 19th century, the realization that TB spread through contaminated milk led to the invention of batch pasteurization, a low‑temperature technique still used to safeguard dairy today.
3 Malaria
Malaria is a mosquito‑borne disease caused by parasitic protozoa, producing flu‑like symptoms that can prove fatal. It remains one of the world’s deadliest killers, infecting over 200 million people in 2016 and claiming nearly half a million lives.
Beyond its health toll, malaria may have played a role in the death of Alexander the Great. More significantly, the disease shaped the transatlantic slave trade: malaria was absent in pre‑colonial America, but African populations, having survived childhood infections, possessed partial immunity, making them the preferred labor force for Southern plantations.
This forced migration spread malaria across the New World, decimating indigenous peoples and fueling the brutal slave economy.
Scientifically, malaria propelled the study of disease vectors, teaching researchers how insects transmit pathogens and how to disrupt those cycles.
2 Ebola
Ebola, short for Ebola hemorrhagic fever, emerged in Africa in the late 1970s. The virus causes severe internal and external bleeding, accompanied by sore throat, muscle pain, vomiting, diarrhea and, in many cases, death.
Depending on the strain, mortality can hover around 50 % but may soar to 90 %. The deadliest outbreak began in West Africa in March 2014, killing five times more people than all previous Ebola episodes combined.
Cases spilled over to the United States and several European nations, including the United Kingdom, France, Germany and Spain. Full containment was not achieved until 2016, testing the World Health Organization’s pandemic response capabilities.
1 Cholera
At its most lethal, cholera can turn from a silent infection to a fatal one in under three hours. The disease spreads through contaminated water or food, thriving where sanitation is poor. Originating in the Ganges Delta of India, cholera has since launched pandemics across South Asia (1961), Africa (1971) and the Americas (1991).
Each year, up to four million people contract cholera, and more than 100 000 die. In 2010, the United Nations declared clean drinking water a human right, a milestone directly tied to combating water‑borne pathogens like cholera.
Evan Beck, a freelance writer based in the San Francisco Bay Area, contributed to this roundup.