Human Immunodeficiency Virus (HIV) is a virus that attacks key parts of the immune system. It attacks the T-cells also known as CD4 cells, which normally fight infections and disease. HIV invades these cells, uses them to self-replicate, and then destroys the host cells. Over time, HIV can destroy enough of the CD4 cells that the body can no longer fight infections or diseases. At that point, HIV infection can lead to Acquired Immunodeficiency Syndrome (AIDS).
AIDS is the final stage of HIV infection. People with AIDS have compromised immune systems, which put them at increased risk of opportunistic infections. AIDS is diagnosed when the patient has one or more specific opportunistic infections, certain types of cancers, or a very low number of CD4 cells.
HIV is spread via bodily fluids, such as: blood, semen, rectal fluids, vaginal fluids, or breast milk. The fluids of an infected individual must come into contact with mucous membranes, damaged skin, or directly into the bloodstream to pass the infection to another person. HIV is contracted via intercourse or sharing needles. HIV cannot be spread in the air, water, insects, casual contact, toilet seats, saliva, tears, or sweat. HIV that infects humans can also not be carried by animals.