The human immunodeficiency virus (HIV) is a virus that attacks the body's immune system. If HIV is not properly treated, it can lead to acquired immunodeficiency syndrome (AIDS). Studies show that HIV may have jumped from chimpanzees in Central Africa to humans as far back as the late 1800s. The chimpanzee version of the virus is called simian immunodeficiency virus. It was probably passed to humans when they hunted these chimpanzees for meat and came in contact with their infected blood. Over decades, HIV slowly spread across Africa and later into other parts of the world. The virus has existed in the United States since at least the mid to late 1970s.
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