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The human immunodeficiency virus (HIV) is the virus that causes acquired immunodeficiency syndrome (AIDS), leading to the progressive destruction of the immune system.

An estimated 34 million people have contracted HIV since 1980 and around 30 million people have died from AIDS related illnesses.

HIV targets T cells, which are specialised white blood cells with a regulatory role in the immune system.

HIV infection ultimately causes a huge drop in the number of T-cells circulating in the blood stream. When the number of T cells drops below a certain level, the immune system becomes compromised and the person develops AIDS.

HIV is a virus that has emerged within the last century. It is thought to have evolved from a virus that infected monkeys to a virus that can infect humans. Emerging viruses are a significant challenge to science and medicine.