The human immunodeficiency virus (HIV) is virus responsible for causing the condition known as acquired immunodeficiency syndrome (AIDS). It belongs to the Retroviridae family and to the subfamily of lentiviruses, which include the simian immunodeficiency virus and bovine immunodeficiency virus. There are two forms of the human immunodeficiency virus known to cause AIDS in humans, HIV-1 and HIV-2. Human immunodeficiency virus 1 is the most common type, generally causing AIDS in many parts of the world including Europe and the United States, while HIV-2 had been isolated from patients in India and West Africa.
Infection with the human immunodeficiency virus usually affects the immune system and the central nervous system of patients. The virus, once it enters the body, is capable of binding and entering the cells of the immune system, particularly the CD4+ T cells. These cells are important in the body's defense against infection. Destruction of CD4+ T cells is compensated for by continuous production of these cells during the early course of the disease, and many people infected with HIV may show no symptoms for up to seven to ten years.
HIV antibody testing, however, such as the enzyme-linked immumosorbent assay (ELISA), can give a positive result as early as two to six weeks from day of infection. As the virus continues to multiply inside the body, the CD4+ T cells eventually decline in number, allowing infections to develop. Symptoms associated with the human immunodeficiency virus infection range from mild to severe, and include swollen lymph nodes, fever, muscle aches, headaches, and diarrhea.
AIDS is the final stage of HIV infection. The CD4+ T cells of some AIDS patients can even decline to 200 cells per mm3 or less, from a normal value of 800 to 1000 cells per mm3 or higher in individuals without the infection. Symptoms of AIDS often include fever, extreme tiredness, diarrhea, unexplained weight loss, swollen lymph nodes in many parts of the body, as well as frequent infections with many types of bacteria and fungi at the same time. Neurological symptoms are also present, like loss of memory and depression. Cancer, like Kaposi sarcoma and invasive cervical cancer, can also develop in AIDS patients.
Unprotected sexual intercourse with an infected person is the most common mode of transmission of the human immunodeficiency virus. Sharing needles and syringes, as well as transfusion of infected blood, have been blamed for the spread of the disease in other people. Infected mothers can also transfer the virus to their unborn child while in the uterus, and to their infants during birth as they pass through an infected birth canal, or through ingestion of breast milk. The virus cannot be transmitted to other people by casual contact, like kissing and sharing utensils, or through mosquito bites.