The body's immune system fights illness and infection. When a harmful organism, like, a virus or bacteria, enters the body, it is attacked by the immune system. The immune system makes antibodies from special blood cells to fight the infection.

The first time an organism enters the body the immune system is not ready for it. It takes time for the body to make antibodies. During this time the person gets sick. As the antibodies are made the person gets better.

The immune system then remembers that organism. If it enters the body again, the infection is usually controlled before the person gets sick, because the antibodies are already there. The body is immune to the disease.

Immunisation is when a vaccine is given to the person. It gets the body to make these antibodies before getting sick.