A virus is a type of particle that can invade your body and make you sick.

Specifically, they invade cells—the tiny units your body is made of.

Viruses are different from bacteria.

That’s why antibiotics, drugs that treat bacterial infections, don’t work on viruses.

There are even viruses that infect bacteria!

Viruses are as different as the illnesses they cause.

But these tiny packages all have a few things in common.

They all have genetic information wrapped in a protein shell.

Some have an oily covering, called an envelope.

That’s it. But that’s all it takes to make you sick.

The Biology of Viruses

How Viruses Work — Viruses are very simple. This keeps them small, and their tiny size helps them get around. The tradeoff is they can’t do much on their own. To reproduce, a virus has to break into a host cell. Then it takes over the cell, stealing energy and raw materials to make more viruses.

Viruses Are Diverse — Viruses come in many shapes and forms. They differ in the host cells they infect, the symptoms they cause, and much more.

Viruses can change over time

The fit between a virus and its host cell is specific, like a lock and key. It’s why your dog can’t get measles. And why you can’t get distemper but your dog can. Yet viruses don’t stay the same. They can change and evolve—often with big effects.

Why Are Novel Viruses a Big Deal? — Ebola, HIV, and the novel coronavirus called SARS-CoV-2 are viruses that started in animals then spread to people. These and other new, or novel, viruses are challenging for individuals, public health officials, and healthcare workers alike.

When Viruses Jump Hosts — Why do some animal viruses spread to people and others don’t? There are many factors at play, including how specific they are, how they spread, and whether they have a chance to jump hosts.

How Viruses Evolve — The changes in viruses as they evolve have big impacts on human health and society. It’s how viruses adapt to new hosts, evolve into new strains, become resistant to anti-viral drugs, and more.

Stopping the Spread

For as long as there have been people, viruses have been making us sick. Fortunately, we've found ways to slow, and sometimes even stop, the spread of viruses. It takes many tools, which are most powerful when used together.

Lessons Learned From Past Outbreaks — Our biggest successes happen when everyone works together to spot outbreaks and slow their spread. While scientists are important, community members also play a vital role.


The best way to stop a virus is to avoid getting sick in the first place. A virus needs a host to replicate. If we stop it from spreading, we can wipe it off the face of the earth.

Vaccination is by far the safest, most effective, and least costly way we have to block viruses from spreading.

How Vaccines Work — Vaccines teach your body how to fight an infection, but without giving you the actual disease.

Types of Vaccines — Each vaccine is designed to block a specific type of infection. And since viruses are so different, scientists have come up with many approaches for making vaccines.

Herd Immunity — Not only do vaccines protect individuals, they also work at the community level to protect the most vulnerable among us.

FAQs about Immunity & Vaccines

Other prevention tools — Everyone has a role to play in protecting ourselves, our families, and our communities. Even young children can take part. Hand washing, for example, is a simple habit that helps prevent the spread of many viruses. Physical distancing and other measures can help put space between people who may be infected and those who are healthy. This lowers the chance of spreading the virus.

Responding to Outbreaks

Treating Viral Infections — When people do get sick, drugs and other treatments can help them get better. Most treatments are aimed at either the virus, the immune system, or the symptoms.

Once a virus is spreading, a number of tools can help slow it down.

  • Diagnostic tests detect whether someone is infected. If infected, a person can get treatment and take extra steps not to spread the virus further. Usually diagnostic tests look for genetic information, viral proteins, or antibodies (a type of protein the immune system makes to fight viruses).

  • Contact tracing helps identify people who have been exposed to the virus. Contact tracers can let them know they’ve been exposed and tell them how to be safe.

Pandemic Preparedness

New viral outbreaks will continue to happen. It’s important to be ready.

Predicting an outbreak or catching it early can help to keep it from spreading. That’s why most countries have surveillance programs that look for unusual symptoms and patterns. The faster we spot a new outbreak, the sooner we can focus on stopping it.

There's Still Much to Learn

The viruses that make people sick get the most attention. But compared to all the viruses out there, their numbers are small.

Viruses make up a huge part of the living material on Earth, and they keep ecosystems healthy. Without them, our planet would likely be unable to support life. Some may even help keep our bodies healthy.

We know enough about viruses to take plenty of steps to protect our health. But there’s still a lot we don’t know. As research grows, so will the ways we can protect ourselves.