You are not going to make it through life without getting sick. It happens to the best of us. What kind of sickness you end up with depends on a number of factors. Some illnesses are far easier to get over than others. Some are pretty much death sentences the moment you’re diagnosed.
The cause of your illness can also vary greatly. Did you eat something that had gone bad? Chances are you picked up some kind of bacteria. Did you get sick after going swimming in some polluted water? You might have got a parasite. Did somebody sneeze on your bagel? You might have a virus. And let’s not forget that, if there’s mold growing in your house, you could be infected with a deadly kind of fungus. What a fun group!
In an ideal world, you’ll just avoid every kind of dangerous bacteria, parasite, fungi, and virus. But the world is rarely ideal. So what’s the most dangerous one of them all? If an evil wizard trapped you in a room with four doors, which would be the one that you’d be most likely to survive a walk-through? Let’s look!
The Basics
In general, a virus is more dangerous than a bacterium. Take that with a grain of salts, of course, because circumstances will vary. The virus that causes a cold is probably not going to be as dangerous to you as botulism.
Bacteria are single cells, and they’re able to survive on their own. Most bacteria are harmless, some are even helpful. Your gut has 100 trillion bacteria in it right now to help you digest your food. Only a small number of them are actually going to cause you any harm. Bacteria can be 10 to 100 times larger than viruses, or about one to three microns in length. Salmonella is a common bacterium.
On the other hand, viruses don’t do well on their own. They have a parasitic nature and require a host to help them survive. They need your body to reproduce and proliferate the cost of which is you getting sick and maybe dying. Viruses may be as small as 20 to 200 nanometers in diameter.
Parasites, part of a group called eukaryotes (meaning their cells have a nucleus and internal structures) are larger than viruses and often bacteria as well. Some parasites can be whole, living organisms, like a tapeworm.
Fungi are most often found in the form of spores and molds. Athlete’s foot is a kind of fungal infection.
Bacteria Breakdown
A single bacterium is a single cell. It is one complete little microorganism all on its own and can live outside of a human body. In fact, many bacteria happily reproduce in the soil, in rotten food, on your skin and anywhere else the conditions are right.
The dangerous kinds of bacteria can affect your body in a number of ways. Many dangerous bacteria are able to produce toxins which can be deadly and are what lead to serious infections in the body. The toxins can paralyze the cells in your body or even destroy them, disrupting normal cell function and causing serious damage. Others can reproduce so prolifically that they crowd out your normal, healthy cells.
Thanks to antibiotics, medicine has been able to save countless lives by treating bacterial infections. Antibiotics can either kill or slow the growth of bacteria. They do this by either destroying the bacteria cell wall or limiting its ability to grow and reproduce.
Because bacteria are able to reproduce very quickly, some every 20 to 30 minutes, they’ve also been able to mutate quickly. This has given rise to antibiotic-resistant bacteria. Over time, the bacteria that has survived has mutated to develop various defense mechanisms against antibiotics. Some may produce enzymes that destroy antibiotics, and others have ways of removing the antibiotic before it can reach its target.
Some more common bacteria, like salmonella, gonorrhea, and campylobacter, have developed strains that are resistant to antibiotics. This means that any infection that may have been considered easily treatable could become far more dangerous and deadly as it evolves.
Because of the ever-changing nature of bacteria, it’s hard to choose the “worst” of them all. In 2024, the World Health Organization issued a list of dangerous, drug-resistant bacteria and there were 15 chosen. Near the top of the list were bacteria that cause tuberculosis, called mycobacterium tuberculosis. This is potentially the deadliest bacterium in the world and is responsible for 1.7 million deaths per year.
Virus Breakdown
Viruses are not cells or living organisms on their own. Instead, a virus is a small amount of genetic material that is encased in protein. A virus can only work if it’s inside a living organism. On its own, it’s nothing, it has no function or purpose.
Once inside a host organism, the virus uses the cells of that living organism to replicate. This process can end up destroying cells and leading to infections. Because viruses are so small, they can even infect bacteria and fungi. You can breathe them in or contract them through things like mosquitoes in ways that bacterial, parasitic, and fungal infections can’t because they are larger.
While a host body will try to produce antibodies to fight off a virus, if they get overwhelmed, and the virus can replicate faster than the host organism can fight it off, that’s when sickness takes over and the potential outcome is death. The viruses need your cells to reproduce because they don’t have the material or energy to do it on their own. This process destroys your cells.
Your body will fight back by raising the temperature. That’s what a fever is. Viruses tend to need a very delicate balance of temperature to survive and even a few degrees too warm can kill them. Unfortunately, a prolonged fever is dangerous for you as well.
In addition, your immune system will try to use antibodies to fight the virus if it can. It needs to be exposed to a pathogen before it can make antibodies, however. If the virus is something you have never experienced, you will have no antibodies at first and your immune system may not be able to fight back.
A virus like Ebola is extremely deadly. Up to 90% of people who contract Ebola will die from it. While that’s terrifying, it also ends up being one of Ebola’s weaknesses. Because it kills so quickly, the disease is not able to spread as well as others that don’t have such a high, swift mortality rate. Outbreaks, once isolated, tend to burn themselves out before they spread too far from the source.
Judging the danger of a particular virus can be tricky. While Ebola kills up to 90% of people who contract it, it’s not widespread. However, HIV has spread all over the globe and is arguably one of the deadliest diseases in history. As many as 32 million people have died from HIV. That said, new treatments have greatly reduced the overall mortality rate, and the odds of dying from HIV now, with treatment, are very low.
In 1918, a flu pandemic killed somewhere between 50 million and 100 million people.
Rabies is another virus that can seem quite common but is exceedingly dangerous. Without proper treatment, the mortality rate for humans infected with rabies is nearly 100%.
Viruses we have mostly eradicated were far deadlier than what most modern people can understand. Smallpox, for instance, killed about 300 million people.
Fungi Breakdown
Thanks to pop culture, most people are familiar with just how terrifying a fungal infection can be. What first gained attention as quirky articles on the internet were then made into a worldwide phenomenon in the game and subsequent TV show called The Last of Us.
In The Last Of Us, much of mankind has been wiped out by a fungal cordyceps infection. Cordyceps is a real thing that has been seen infecting far less complex organisms like ants. The fungus grows inside of them, literally breaks through their bodies, and forces them to keep moving like zombies even when they should be dead. Fascinating stuff, but not applicable to humans. Our immune systems are vastly more complex than an insect’s and, as a result, we are immune. Unless it mutates one day.
While cordyceps won’t kill you anytime soon, it doesn’t mean other fungi aren’t a danger. In 2023, the CDC warned about the rapid spread of Candida Auris. The fungus is resistant to most antifungal medications and was putting people in the hospital on ventilators. The infection can spread into your heart, lungs, blood, eyes, bones and organs.
Another fungus, cryptococcus neoformans, has a mortality rate between 41% and 61% and is especially dangerous to those with an already compromised immune system. The fungus, a kind of yeast, is found all over the world in soil. It can cause a kind of meningitis.
Aspergillus fumigatus is a kind of mold and has a mortality rate as high as 90%. You can find it almost anywhere that leaves fall on the ground and start to rot. Estimates suggest all of us inhale between 10 and 100 Aspergillus spores every day.
The problem with Aspergillus and other fungal infections is they receive less attention and fewer resources than bacteria and viruses. Also, fungi are quick to adapt to medications and become resistant. Nevertheless, as many as 1.7 million people per year die from fungal infections, which is more than malaria and twice the number who die of breast cancer. There are over 150 million severe infections reported that are damaging but not fatal, as well.
Parasite Breakdown
Parasites are, hands down, the creepiest and most disturbing things that can infect you. Even if they aren’t as deadly as other infections, they tend to be more off-putting if for no other reason than many of them are big. These are living organisms that take up residence inside of you. The way they get inside of you can be just as disturbing as the fact they are inside you.
Take Strongyloides, for instance. This parasite transmits through feces but can live in soil for weeks. If you walk barefoot across it, they will burrow through the flesh of your foot. They’ll ride your bloodstream all the way to your lungs and then cause you to cough. Coughing brings them to your mouth where they get swallowed into your gut, right where they want to be. They can live for years in there and may turn deadly depending on the medications you take.
Giardia, a small parasite, is transmitted most often through feces or things contaminated with it. You can get it on your hands or in the food you eat and ingest it without realizing, as it’s a small, one-celled organism.
Tapeworms, which can infect you if you accidentally ingest their eggs in undercooked meats, or from meat handled unhygienically, can grow to be up to 12 feet long in your gut, but some have been reported to be over 50 feet. They can live for 30 years.
Brain-eating amoebas have been contracted by people swimming in still, warm bodies of water like ponds in the US. They enter through the nose and infect the nervous system with a near 100% mortality rate.
Parasitic infections can lead to sepsis among numerous other symptoms. One of the most well known and deadly parasites is the malaria parasite, transmitted by mosquitoes, which caused over 600,000 deaths in 2022.
As disgusting as parasites can be, many of them will not actually kill you. It’s not in a parasite’s interest to kill its host, after all. It’s estimated about one in seven people in the world currently have an intestinal parasite. Some estimates boost that to about half the world’s population.
So Which is Worst?
You can’t make a strong claim that any one of these deadly pathogens is better than another. There is far too much variety among each category to conclude one is preferable to another. In addition, there are so many mitigating factors that can alter just how dangerous an infection from one of these pathogens could be.
It’s much easier to state that you really don’t want to get an infection of any kind, be it parasitic, fungal, viral or bacterial. None of these are going to be a good time for you, and all should be avoided or treated quickly if they ever come up.