How do viruses infect cells?

What is a virus?

Before talking about how viruses work, let’s take a look at what a virus is, exactly. A virus is a kind of infectious particle, made of genetic code in a protective shell. Most scientists don’t think of viruses as living things, because they can’t reproduce without help from a living cell. They need to invade a cell in order to make more viruses.

Viruses like the rhinovirus shown here (which causes the common cold) don't have a fatty envelope. This makes them very different from enveloped viruses, like the flu. Colorized image by José R Valverde from Pixabay.

There are many kinds of viruses, and viruses infect every form of life on earth. Plants, animals, fungi, and even bacteria can be infected by viruses. One kind of virus usually only infects one kind of host. So, a virus that makes your cat sick isn’t likely to make you sick. But very rarely, a virus can mutate in a way that allows it to cross from one kind of host to another. HIV, bird flu, SARS, and COVID-19 are examples of this.

The most basic parts of a virus are genetic material (DNA or RNA), a few proteins that will help the virus to replicate in a cell, and a protective protein shell. Some viruses also have a lipid envelope that is similar to the barrier that surrounds our cells. Viruses with a protective envelope are called enveloped viruses. Those without one are called non-enveloped (naked) viruses.

How do viruses work?

Viruses can’t make new viruses on their own. Instead, they take over cells, and trick the cell into making new viruses. To enter the cell, a virus floats up to, or lands on a cell, then attaches to a receptor. Receptors are proteins on the surface of cells that act like locks. They will only fit a specific key. The sneaky virus has a copy of that key. Proteins on the virus’ surface are shaped just like the keys, and fit into a receptor. This starts a process that leads to the virus either entering the cell whole, or injecting its DNA or RNA into the cell.

Once a virus enters the cell, it can use the cell to make more viruses. The virus can do this because viruses and cells have an important thing in common: they both use DNA and RNA. DNA and RNA are molecules that act like instructions. Viruses bring their DNA and RNA instructions to the cell, and trick the cell into following them. The cells follow the virus’ directions and make all the necessary parts for the virus. Cells even use their own tools and raw martials for the virus parts. New copies of viruses can then be put together inside the cell. Eventually, the new virus particles escape the cell, often killing it. These new viruses go on to find more cells to infect.

Martinus Beijerinck was the scientist who gave these infectious particles the name “virus”. He wasn’t sure yet what viruses looked like, just that they were much smaller than bacteria, so he thought a virus was some sort of toxin.

In humans, viruses that cause disease like cold and flu are spread through bodily fluids, like spit or snot. The virus is so small that it leaves our bodies in these fluids, and can even float through the air in droplets from a sneeze or cough. The virus can enter the body through the eyes, nose, or mouth. It can also land somewhere and wait. When someone else touches it, then rubs their face, the virus can be passed on to the new person.

Pathogen protection

When a virus enters your body, your immune system eventually finds it. It raises your temperature to help fight the invaders, makes your nose run to trap the virus in snot, and attacks the virus particles directly. Scientists and doctors have also figured out ways to help the body battle these pesky germs. One way is to get vaccinated against a virus. A vaccine will train your immune system to recognize a virus as soon as it enters your body, before it can take over. Antivirals are another method to help you fight the virus. These are medicines that help prevent infection from happening or fight the virus once you are already infected. They can stop the virus at many phases of its “life” cycle, including when it is entering your cells, making new viruses inside your cells, or leaving your cells to infect new ones.

A flu vaccine being given to military personnel through the nose.

But, we don’t have vaccines or antivirals for all viruses. That’s why washing your hands with soap and water is a great way to protect yourself from viruses getting into your body in the first place. Soap breaks down the protective lipid envelope of some viruses. It also is good at trapping and washing away naked viruses, and bacteria too. It works better than disinfectants like hand sanitizer. But you can use hand sanitizer when you think you’ve touched a virus, but can’t wash your hands.

If you’re sick with a virus, it’s important to protect others. Sneeze into your elbow or a tissue to keep particles from floating in the air. Throw your tissues in the trash right away and wash your hands. Stay home and rest when you’re sick. You will heal up faster, and won’t give your friends your germs.

Virus illustration by Arek Socha from Pixabay.