Within eighteen months of the preparation of the first edition much has happened in the AIDS field. The number affected are still increas­ at an alarming rate - these are now (at December 1986) almost ing 30000 cases in the United States and over 600 cases in the United Kingdom. Worldwide the AIDS pandemic is a most serious disease threat and now affects over 80 countries. It is estimated that for every AIDS patient there are about 50 other persons infected with the AIDS virus. AIDS threatens every country and every society. The main pre­ ventative measures of health education to prevent the spread of AIDS must be taken by millions of individuals helped by governments and voluntary organizations. What is clear is that AIDS and AIDS­ related conditions will be with us, in our hospitals and in our com­ munities for many years to come. It has been well established that the agent that causes AIDS is a virus called human immunodeficiency virus (HIV). People can catch HIV through sexual contact or by infected blood from infected needles and syringes, from infected mother to unborn child, or treatment with contaminated blood or blood products.