How viruses make us sick
A virus is the simplest of germs—it is nothing but genetic material encased in protein. Researchers debate whether a virus is even "alive."
By itself, a virus can accomplish nothing—it needs to enter a living thing to perform its only function, which is to replicate. When a virus gets inside a human body, it can hijack a person's cellular machinery to produce clones of itself, overtaking more cells and continuing to reproduce.
How to treat viruses
Since each virus is very different, no one drug exists to attack whichever virus is in your body. Vaccines give preemptive protection from certain viruses by training the body's immune system to recognize and attack a specific virus.
How bacteria makes us sick
Bacteria are bigger and more complex than viruses, though they can still spread through the air. A bacterium is a single cell, and it can live and reproduce almost anywhere on its own: in soil, in water and in our bodies.
For the most part, we live peacefully with bacteria—the colonies in our guts are helpful to us and strengthen our immune system. But like viruses, bacteria can also harm us by replicating quickly in our bodies, killing cells. Some bacteria also produce toxins which can kill cells and cause an outsized, damaging immune reaction.
How to treat bacterial infections
Broad-spectrum antibiotics were developed to kill bacteria in our bodies and in the food supply by inhibiting their growth.
How fungi makes us sick
Fungi are more complicated organisms than viruses and bacteria—they are "eukaryotes," which means they have cells. Of the three pathogens, fungi are most similar to animals in their structure.
There are two main types of fungi: environmental, which are yeast and mold that often live in soil and don't generally cause infection in most healthy people; and commensals, which live on and in us and generally don't hurt us.
Commensal fungus, may play a beneficial role in our overall health.
Certain environmental fungi reproduce "spores," particles that can enter our body through the lungs or on the skin. These fungi can be especially damaging for people with weakened immune systems, as the fungi can spread quickly and damage many organs.
How to treat fungal infections
Fungi are slower to mutate, so they are easier to target with antifungal medications than bacteria are with antibiotics.