Viruses

Smaller than small

Viruses are tiny particles that cause diseases like colds, flu, chickenpox and COVID-19. They are much smaller than bacteria; most can be seen only with an electron microscope. About 1000 viruses could fit across the width of one bacterium… and a bacterium is already tiny.

Are viruses alive?

Scientists still debate this. Viruses have some features of life but not others.

Outside a cell, a virus is inactive, like a seed waiting to sprout.

What a virus is made of

Viruses come in many shapes: spheres (flu), rods (tobacco mosaic virus), and even shapes like lunar landers (bacteriophages, which infect bacteria).

How a virus makes copies

1. Attach: the virus locks onto a host cell, like a key in a lock.
2. Enter: it injects its genetic material, or the whole virus is taken in.
3. Take over: the viral genes order the cell to make viral parts instead of its own.
4. Assemble: new viruses are put together.
5. Release: hundreds or thousands of new viruses burst out or bud off, often killing the cell, and go on to infect more cells.

Each virus usually infects only certain kinds of cells. Cold viruses attack the cells lining your nose and throat.

Viruses and disease

Viruses infect all kinds of living things. Plants get viral diseases, and bacteriophages infect bacteria.

How the body fights back

Your immune system defends you against infection.

1. Barriers like skin and mucus keep many viruses out.
2. White blood cells attack infected cells.
3. The immune system makes antibodies, proteins that attach to a specific virus and block it.
4. Memory cells remember the virus, so if it returns, your body responds faster. This is immunity.

A fever is part of the fight: higher temperatures can slow some viruses down.

Vaccines

A vaccine teaches your immune system to recognize a virus without making you sick. It may contain a weakened or killed virus, a piece of the virus, or instructions (mRNA) for your cells to make a harmless viral protein.

Vaccines have prevented huge numbers of deaths. Smallpox, which killed hundreds of millions of people, was declared eradicated in 1980 thanks to a worldwide vaccination campaign. Polio has been eliminated from most of the world.

Treating viral infections

• Antibiotics don’t work on viruses. They target bacteria.
• Antiviral medicines can treat some viruses, like flu, HIV and COVID-19.
• For most colds, the body clears the virus on its own; rest and fluids help.

Stopping the spread

• Wash hands with soap for at least 20 seconds.
• Cough or sneeze into your elbow.
• Stay home when sick.
• Get recommended vaccines.

Common mistakes

• Thinking antibiotics kill viruses.
• Thinking viruses and bacteria are the same. Bacteria are cells; viruses are not.
• Thinking all viruses are harmful. Some infect only bacteria, and scientists use viruses to deliver gene therapies.

Try it yourself

1. Put glitter on your hands as pretend "viruses." Shake hands with friends and touch objects. See how far it spreads. Then try removing it with water alone, then with soap.
2. Draw the five steps of a virus taking over a cell as a comic strip.
3. Make a table comparing bacteria and viruses.

Quick recap

• Viruses are tiny particles of genetic material inside a protein coat.
• They can reproduce only inside a host cell.
• They cause diseases in people, animals, plants and even bacteria.
• The immune system fights viruses with antibodies and memory cells.
• Vaccines prevent viral diseases; antibiotics don’t work on viruses.