Human immunodeficiency virus, or HIV, belongs to a family of organisms known as retroviruses, which insert their own genetic code into cells for the purpose of replication. When a person contracts the virus, it enters the host's cells, especially CD4 T-cells, which help to fight disease and whose presence can signal the development of HIV/AIDS in the patient.

Studies show that HIV may have jumped from apes to humans as long ago as the late 1800s, and a chimpanzee in West Africa is regarded as the initial source of human infection.

Scientists believe the chimp had simian immunodeficiency virus, which was most likely transmitted to humans and mutated into HIV through infected blood when the animal was hunted and eaten.

The virus first spread across Africa, and then to other parts of the world in the subsequent decades. China detected its first cases in 1985.

HIV contains ribonucleic acid, which it transforms into DNA using an enzyme called reverse transcriptase. The resulting DNA integrates with the human genome in the cell, fooling it into replicating the virus.

HIV is prolific and can create trillions of copies of itself in a short period of time. The copies may remain quiescent in the genome or they may replicate.

During times of peak viral reproduction, just 1 milliliter of blood can contain more than 1 million copies of the virus, many of which differ in minor ways from the original and may be resistant to a wide range of medications.

Source: The Chinese Center for Disease Control and Prevention