Vaccines are effective tools that work with the body's natural defences to develop immunity to disease. To understand how vaccines work, it is useful to first consider how the body fights illness. When bacteria or viruses invade the body, they multiply and attack healthy cells. The invasion, an infection, produces illness. The body's immune system responds to infection by producing antibodies—protein molecules that fight the antigens, or invaders, and protect the body from the same infection in the future. Regrettably, the first time the body encounters certain bacteria or viruses, it can take too long for the antibodies to develop. Infections from measles or whooping cough, for instance, can spread and kill a person before the immune system can react. Vaccines, however, can be used to prepare the body to meet these invaders in advance of a full-blown infection. By introducing a version of the antigen that is too weak to cause any infection into the body, the vaccine activates the immune system to begin the production of antibodies. Once the threat of illness is eradicated, many of the antibodies will break down, but the immune system retains some 'memory cells' that allow it to recreate the antibodies quickly if it encounters the antigen again. The benefit of vaccines is also felt at the community level. Some individuals cannot be vaccinated because they are too young or because their immune system is not robust enough. If everyone around them is vaccinated, however, those who have not been vaccinated are protected by the so-called 'herd immunity'. Put another way, these individuals are unlikely to encounter the disease, so their likelihood of infection is very low.