FAQs About Viruses

Where did viruses come from?

Right now, we don’t have a clear answer, though many scientists are working on it.

It’s possible viruses came about in more than one way. Some may have been the first life. Others may have evolved from bacteria. It’s hard to figure out because the usual ways of looking into the past don’t work for viruses. Viruses don’t leave fossils. And their genetic information changes too quickly to reveal much about their origins or distant past.

Are viruses alive?

This question is an ongoing scientific debate. The short answer is: it depends on who you ask.

On the “yes” side, viruses have a lot in common with all life forms we’ve ever known. They have genetic information and proteins. And like all living things, viruses reproduce and evolve.

Yet on the “no” side, viruses can’t do much of anything on their own. Other parasites use a host for nutrients or a place to live. But viruses lack the cellular parts needed to sustain life. They rely on a host to give them energy and raw materials, and for reproduction.

In the end, it comes down to how you want to define life. And defining exactly what it means to be alive gets pretty philosophical. But, regardless of whether they’re alive or not, viruses are tiny particles with a big impact on our lives.

Why do the same viruses cause different symptoms (or even no symptoms) in different people?

There are a lot of reasons. Each person has a different mix of factors that can make their symptoms better, worse, or just different. Some of the most important ones are listed here:

Exposure Level: When it comes to severity, people who are exposed to (i.e., come into contact with) a higher dose of virus tend to get sicker.

For viruses that spread through droplets that people breathe out (like SARS-CoV-2), wearing a mask means you breathe in fewer virus particles. That’s why it’s a good idea to wash your hands and mask up!

It also matters where the virus enters your body. The same virus can give you pink eye (conjunctivitis) or a runny nose, depending on where it lands.

Exposure is one of the only factors we have some control over. Identifying viruses you’re at risk for and learning how to reduce exposure is a way to be proactive about your health.

Partial Immunity: Past exposure to one virus can sometimes give you protection against a new one. If the surface proteins on the two viruses look alike, your immune system may be able to recognize both.

An example is influenza A, which causes the flu. The virus comes in many strains, but different strains can have some proteins in common. If you develop an immune response against these proteins in one strain, you may have some protection against others. Even a partial immune response can be helpful. It decreases the number of viruses in your body, making the infection less severe (see Exposure level above).

Underlying Health Conditions: Long-term health problems are risk factors for severe symptoms from many viruses. Common examples include high blood pressure, heart disease, obesity, diabetes, and chronic kidney disease.

It depends on the virus which health conditions have the biggest impact. Impacts tend to be greater when the condition and the virus affect the same organs or systems. For example, the flu and COVID-19 damage the lungs, so they’re extra dangerous for people with respiratory diseases (like asthma, COPD, and chronic bronchitis). And diseases like HIV and cancer cause people to have a decreased immune response. Other connections are less clear and need more research.

Age: All health factors being equal, younger people tend to have a stronger immune response. It tends to get weaker with age. Many parents have likely had the experience where their child is sick for a few days. But when the adult catches the illness, they’re sick for a week.

One thing to note is that while a strong immune response is usually good, too big of a response can make you really sick. A huge response can cause a lot of tissue damage that makes things worse.

Immune response: Due to their age or health conditions (e.g., diabetes, cancer), some people have a weak immune response to infection. This makes it harder for their bodies to clear a virus.

Having a strong immune response is usually a good thing. But one that’s too big can make you really sick. That’s because an over-active immune system can damage tissue and make things worse. One example of this is a “cytokine storm.” To learn more about how a strong immune response can cause damage, visit the Immune System section of Treating Viral Infections.

Genetic Differences: Every human is genetically unique. The differences between us can affect how sick we get for near limitless reasons. For example, people can have slight differences in the shape of the cell surface protein a virus uses to enter their cells. Or a piece of host machinery the virus needs once inside can be slightly different from person to person.

It can be bigger picture differences too. For example, tissue distribution of the cell parts a virus needs, especially cell receptors, can vary. And there are tons of normal differences in genes that affect a person’s immune response.

Are viruses always bad?

No! In fact, we couldn't survive without them.

The viruses that get the most attention are the ones that make us sick. But this is a very small portion of viruses overall (far below one percent). Most viruses don’t infect humans. And many keep ecosystems and food webs healthy and balanced.

Some viruses have more direct and positive impacts on human health. There are virus-based cancer treatments and stem cell therapies. Viruses likely help keep the microbes balanced in our gut. And viruses that kill bacteria can be used as a tool against antibiotic-resistant bacteria.


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