The human body and other organisms use their immune system as a means of staying healthy by fighting germs. Micro-organisms thrive in conditions that are warm and offer a supply of food; the human body offers such conditions. Many of the germs that get into the body are attacked by white blood cells, others by antibodies produced by the immune system. When attacked by a particular bacterium, if the body has encountered it before, the relevant antibodies recognise it and quickly replicate themselves to kill the invading cells. This happens when the human body has built up immunity to a particular bacterium.