Each of us shares our air, food, water and shelter with tiny colonies of microorganisms that include viruses, bacteria and fungi. Most of these miniscule microbes are harmless, but some are pathogens—the kind that can make you sick, such as the novel coronavirus that causes COVID-19.

What makes a virus, like the highly contagious strain now causing a worldwide pandemic, different from other germs, such as bacteria or a fungus? How do they each infect us, and how can we recover from them? 

Dr. Fayyaz Sutterwala, director of Infectious Diseases at Cedars-Sinai, says infections are sometimes difficult to diagnose. Though certain microbes can cause very specific disease, many others can infect any organ and cause similar symptoms and immune response.

"For example, bacteria, viruses and fungi can all cause slightly different forms of pneumonia," he says. "The symptoms are subtly different depending on the type of microbe causing the conditions. As physicians, we evaluate to determine the best tests and treatments for each infection."

Below, we explain some differences among these common germs. 

Common forms: Viruses cause colds and flus, as well as more serious conditions such as HIV/AIDS, Ebola and COVID-19.

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.

Viruses also are capable of infecting any living thing, including bacteria and fungi.

When the virus reproduces faster than the immune system can control it, it begins to destroy cells and harm the body.

Viruses are also the smallest germ, making them generally the easiest to contract—they're so tiny they can spread through the air in a cough or a sneeze. Some viruses also are spread by mosquitoes or through bodily fluid.

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.

Read: Ask a Doc: Do Fist Bumps Spread Fewer Germs than Handshakes?

Common forms: Bacteria cause food poisoning, strep throat and urinary tract infections, as well as infections such as tuberculosis.

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.

Broad-spectrum antibiotics were developed to kill bacteria in our bodies and in the food supply by inhibiting their growth. But bacteria are extremely adaptive and can quickly evolve to evade antibiotics. Bacteria share their antibiotic-resistant genes with each other, meaning more strains generate resistance to the drugs we use.

Read: Unraveling How Gut Bacteria Affect Our Health

Common forms: Fungi are responsible for causing conditions such as yeast infections, valley fever and meningitis.

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.

Other fungal infections can be caused by an overgrowth of commensal fungus.

Fungi are slower to mutate, so they are easier to target with antifungal medications than bacteria are with antibiotics. 

Read: Taming the Boogeymen: When Medical "Monsters" Do Good