Immunity is your body’s defences against bacteria, viruses, fungi, and other substances that are not part of the body or might be harmful. The immune system is a complex network of organs, cells, and cell receptors. The system has two basic aims: to recognize non-self entities in the body and to destroy them. This is achieved by a vast and intricate network of mechanisms, but the overall effect is to keep you healthy.

When a germ invades the body, an immediate, innate response is mounted by immune cells called B and T cells. These cells remember the antigens – the chemical markers – associated with each specific invader, so that when the microbes return they can be recognised and destroyed quickly, before they multiply and make you feel sick. This is how natural immunity works, although it can be quite a slow process, as the invaders have to enter the body and leave again in order for memory cells to form.

Vaccination is a way of boosting the immune system by exposing it to a weakened version of the infectious organism, so that it can learn to recognise and destroy the real thing. But the body’s natural immunity can also be boosted by having a good lifestyle, eating well, avoiding stress, and getting enough sleep. This is because the immune system responds to a whole range of environmental and internal factors, not just to certain diseases or conditions. This type of indirect immunity is known as herd immunity, which helps protect babies for example by ensuring that everyone around them is immune to pertussis (whooping cough).