HIV, or Human Immunodeficiency Virus, is an illness that is caused by a virus and transmitted through blood, breast milk, semen, and vaginal and rectal secretions. The virus most commonly is transmitted when an infected person passes it to a non-infected person through open skin (cuts, needle punctures, skin tears, sores) or mucus membranes (sex, breast feeding). Transmission is best avoided by using condoms and by using clean needles for tattoos, medical procedures, and drug use.
The HIV virus infects T cells (also called CD4 cells), which are a type of white blood cell involved in the immune system. When it infects T cells, it uses them to make tons of copies of the virus. Eventually the new copies destroy that cell and go on to infect more cells. It essentially takes good soldiers and turns them into bad or dead soldiers. The more copies you have of the virus (viral load), the more T cells you destroy. (Medical folks: cringe with me a little bit at the inaccuracy of the pictoral representation of HIV below. Non medical folks: please note that the HIV virus is not bigger than your blood cells, and it does not look like pac-man, and T cells do not look like red blood cells.)
When a person has HIV and either doesn’t know it yet or knows it and hasn’t gotten treatment for it, the virus replicates quickly and their CD4 cell count drops over time.
As the viral load grows and the CD4 count drops, the person starts getting sick. At first they get nonspecific symptoms like fever, aches, headache, and swollen glands, so it sort of feels like the flu. But some people don’t feel sick at all until the disease progresses pretty far.
As the CD4 drops lower, the immune system becomes vulnerable to bacteria and viruses that a healthy immune system can fight off easily. So the patient gets “opportunistic” illnesses. When a patient has a CD4 count less than 200 and also has an opportunistic illness, then we say that they now have AIDS, which is Acquired Immune Deficiency Syndrome.
In the US, it is recommended that all people age 15-65 be screened for HIV. There is an excellent explanation of HIV/AIDS (and is also the source of the images above) here. Tons more information on diagnosis, treatment, and prevention can be found here.