What is a virus?

Viruses are very, very small. A virus is much smaller than one of your cells.

You cannot see a virus with the kind of microscope that you use in school. To see a virus, you need to use a special kind of microscope called an electron microscope.

Viruses are not made of cells. They do not have a cell membrane or cytoplasm. The blue-green outer layer in the photograph is a coat made of protein. There are little pegs on the outside of this coat.

The orange part inside contains a substance called RNA. The RNA is made of little threads that contain a set of coded instructions for making more viruses.

How viruses replicate

Viruses cannot do anything at all on their own. They do not respire, feed, excrete or grow. They are not sensitive and cannot move. Viruses have to get inside a living cell before they can make copies of themselves.

The brown viruses in the photograph are H3N2 influenza viruses. This kind of virus can invade (get inside) cells of birds, humans and other mammals. The viruses get into your body by going up your nose when you breathe in. The little pegs on the virus’s coat help it to stick onto one of your cells and then get inside the cell.

When the viruses are inside the cell, each virus bursts open. The virus forces the cell to copy the instructions on its RNA, and make many new viruses. This is called replication. This kills the cell. Then the new viruses burst out of the dying cell, ready to infect more cells.

These flu viruses are just one of thousands of different kinds of viruses we know about. Each kind of virus has a particular kind of cell that it infects. Some viruses infect plant cells.

In 2019, a new virus appeared. We do not know exactly where it came from, but scientists think it developed in a wild animal and then spread to humans. The new virus is similar to the viruses that cause flu and colds. Its official name is SARS-CoV-2. The illness it causes is called Covid-19. This stands for coronavirus disease 2019. The virus quickly spread all over the world.

Many people get the virus without being ill at all, or just have mild symptoms. But in some people, it causes dangerous illness and even death. Scientists will work hard for many years to find the best ways of preventing this, including vaccination, and drugs to treat Covid-19.