Every day the human body is fighting off 31 by destructive pathogens. A person in good health has natural protection in the form of an immune system which works best against familiar microorganisms which may have been encountered during a previous 32 or passed on by the mother before or after birth.

Vaccination is a way to cause 33 immunisation by introducing a small amount of pathogen into the body—just enough for the body's 34 to react by making antibodies. Passive immnnisation can be used as a way of treating someone who is already sick. Proteins from animal 35 are introduced into the patient to give him the necessary antibodies to fight the disease.

Dr. Edward Jenner observed that people who had suffered and recovered from a serious disease called smallpox did not get it again. He also noted that victims of a milder disease, cowpox, which they caught from 36 , were immune to smallpox. He carried out a successful 37 by deliberately giving a child cowpox in order to make him immune to smallpox.