Our immune system helps us stay healthy even though we are constantly exposed to harmful bacteria and viruses. This system is a network of white blood cells and proteins that constantly surveys our body and, on encountering signs of attack, will start to eliminate the infection. This decision to act must be made with care; a weak response may lead to uncontrolled disease, but over-activity could cause an unwanted response to our own cells, as in auto-immune diseases like arthritis or multiple sclerosis.