How does the body fight off a virus?

Artists' concept of the rhinovirus which causes the common cold The rhinovirus is one of the many viruses that can cause the common cold

Viruses are infectious micro-organisms that require a living host to survive and multiply. When one enters your body, it invades and takes over cells, redirecting them to produce more of the virus.

How do our bodies defend us from viruses?

When our bodies come under attack from a viral infection they launch a sophisticated defence known as 'the immune response'. Our immune system is designed to recognise the cells that make up our bodies and repel any foreign invaders such as viruses.

They do this by using a huge army of defender cells which consist of different types of white blood cell. We make around a billion of them every day in our bone marrow.

How HIV/AIDS overwhelms the body

HIV attacks and kills a type of immune system cell known as T lymphocytes. These T cells are crucial for killing other cells in the body that have been infected with germs.

Without T cells, other immune system cells, such as antibody-making B cells, can't work properly.

If HIV is not treated the number of T cells drops steadily. Eventually, numbers fall to the point that the risk of infection greatly increases. This end-stage is known as AIDS.

Source: Science Museum