Pandemic is a large-scale epidemic affecting more than one country. Acquired immunodeficiency syndrome (AIDS) was first diagnosed clinically in the early 1980s, but retrospective diagnosis suggests that it existed well before this date. AIDS is caused by the human immunodeficiency virus (HIV), of which there are several strains. AIDS is a syndrome (a set of signs and symptoms) that develops from a reduced ability to fight diseases, that is, it undermines the body's defenses against viruses, infections and malignant tumors. According to the UN, at the end of 2004, 39.4 million people worldwide were living with HIV/AIDS, 4.9 million people contracted it and 3.1 million people died from it. Approximately 95% of people living with HIV/AIDS live in developing countries. In 2004, 65% of those affected lived in Africa, south of the Sahara. The infection rate is estimated at 8% of all adults compared to 1% globally, and more than 13 million Africans have lost their lives to HIV/AIDS since it was identified. It has become the leading cause of death in Africa, killing more people than malaria and war. Global Spread of HIV/AIDS Medical geographers have concluded that HIV/AIDS spread in a hierarchical pattern of spread from a hotbed area in Central Africa in the late 1970s. The virus initially appeared almost simultaneously in major metropolitan areas in North and South America, the Caribbean and Europe. These areas then acted as hubs for localized spread of the virus, which then spread to major metropolitan areas in Asia and Oceania and larger provincial cities in North and South America, the Caribbean and Europe. Later, in this cascading diffusion model, came the provincial cities of Asia and Oceania and the small towns of North and South America, the Caribbean and Europe. The geography of HIV/AIDS in Africa varies from country to country, from region within the country and from social group. The highest infection rates were in East Africa in the early 1980s, but have now moved to southern Africa, particularly Botswana, Zambia, Zimbabwe and South Africa, where more than 20% of adults are infected..