Acquired Immune Deficiency Syndrome (usually referred to as AIDS) is a syndrome of clinical manifestations caused by long-term infection with the Human Immunodeficiency Virus Type 1 (HIV-1). HIV infection alone, with no pathology, does not constitute AIDS. There must be specific clinical presentations, or laboratory values, or both, coexisting with the presence of HIV in the patient.
- CD4 count less than 200 or
- CD4 percentage less than 14% of total lymphocytes or
- Any of the following illnesses: pulmonary TB, recurrent pneumonia, invasive cervical cancer.
- Expanded definition including 23 clinical conditions published elsewhere
The World Health Organization uses a different staging method for HIV disease. 
The history of AIDs has been detailed.  The illness was first recognized in 1981 as homosexual men presented to their doctors with a rare lung disease called Pneumocystis carinii pneumonia (PCP).
Early on, as researchers struggled to determine the cause of the disease, several names were used by researchers and the press, including "gay-related immunodeficiency" (GRID) and "gay cancer", referring to the classic symptom of Kaposi's sarcoma. HIV-1, at that time called human T-lymphotropic virus type III (HTLV-III), was first isolated from an infected patient in 1983.
After infection with HIV, the virus quickly replicates in lymphatic tissue and travels through the blood to infect most areas of the body. The largest consequence of this is the "hijacking" of certain immune cells, especially so-called T-Helper, or CD-4, cells. Early in the infection, the immune system holds the infection at bay, often causing "Acute Retroviral Syndrome", a flu-like illness. Patients often have swollen lymph nodes. After approximately 8-10 years of infection, the immune system begins to lose its battle. This is primarily seen in the loss of numbers of CD4 cells, however the changes are somewhat more complex. When the CD4 cell level drops low enough, AIDS becomes apparent. The first symptoms of the illness are generally night sweats, weight loss, and oral thrush.
Opportunistic infections (OIs)
The hallmark of AIDS is the appearance of opportunistic infections, meaning infections with organisms that do not usually cause human disease, unless given the right "opportunity". This opportunity is the reduction in cell-mediated immunity, first seen in patients with certain cancers or on anti-rejection drugs for organ transplantation. These infections include:
- Pneumocystis jiroveci, previously Pneumocystis carinii
- Candida albicans (the cause of thrush and vaginal yeast infections)
- Staphylococcus aureus (primarily causes skin infections)
- Toxoplasma gondii
- Streptococcus pneumoniae (the primary cause of pneumonia in AIDS patients)
- Mycobacterium tuberculosis (TB)
- Mycobacterium avium complex (atypical mycobacterium)
- Cryptococcus neoformans (a cause of meningitis)
- Epstein Barr virus (leads to a type of lymphoma)
- Human herpesvirus-8 (causes Kaposi's sarcoma)
The earliest known case in a human was in 1959 from the Congo. Scientists have identified a type of chimpanzee in West Africa as the source of HIV infection in humans. The virus most likely jumped to humans when humans hunted these chimpanzees for meat and came into contact with their infected blood. Over several years, the virus slowly spread across Africa and later into other parts of the world.
HIV is a fragile virus. It cannot live for very long outside the human body. As a result, the virus is not transmitted through day-to-day activities such as shaking hands, hugging, or a casual kiss. Transmission can not occur from contact with toilet seats, drinking fountains, doorknobs, dishes, drinking glasses, food, pets, or insect bites.
HIV is primarily found in the blood, semen, or vaginal fluid of an infected person, and is transmitted in 3 main ways:
- Having unprotected sex (anal, vaginal, or oral) with someone infected with HIV
- Sharing needles and syringes with someone infected with HIV
- Being exposed (fetus or infant) to HIV before or during birth or through breast feeding
HIV also can be transmitted through blood infected with HIV. However, in developed countries, the risk for HIV infection through the transfusion of blood or blood products is extremely low, as supplies and/or donors are screened for infection. Currently, HIV/AIDS kills approximately 2-3 million people per year, primarily in developing countries. Currently in the U.S., there are approximately 500,000 people infected with HIV.
HIV is primarily spread by sexual contact and intravenous drug use. Most early infections in the US were via homosexual sex, and to a lesser extent via intravenous drug use and blood transfusions; most current infections in the world are via heterosexual contact and vertical transmission from mother to child. Mothers infected with HIV transmit the virus to their baby in utero, during childbirth. Mother-to-child transmission can be significantly reduced by the proper use of antiretroviral agents.
Less commonly, contact with infected blood causes HIV transmission. This can occur in health care providers (HCPs) or others exposed to infectious bodily fluids. Transmission is facilitated by breaks in the skin or direct contact with mucosal tissues, such as those found in the eyes, mouth, anus, or vagina. Early in the epidemic, blood transfusions were a significant source of HIV transmission.
Other less likely means of transmission exist, though are rare. There are no confirmed cases from contact with the saliva, sweat or tears of an infected person.
Early on in the epidemic, there was much confusion about the transmission of HIV. After decades of study, this has been clarified. HIV can be found in various body fluids, however its highest concentrations are found in semen, blood, and vaginal secretions. It can also be found in breast milk, but recent research  shows that exclusive breastfeeding tends to protect against HIV transmission.
As summarized by the Centers for Disease Prevention and Control:
An exposure that might place HCP at risk for HIV infection is defined as a percutaneous injury (e.g., a needlestick or cut with a sharp object) or contact of mucous membrane or nonintact skin (e.g., exposed skin that is chapped, abraded, or afflicted with dermatitis) with blood, tissue, or other body fluids that are potentially infectious. In addition to blood and visibly bloody body fluids, semen and vaginal secretions also are considered potentially infectious. Although semen and vaginal secretions have been implicated in the sexual transmission of HIV, they have not been implicated in occupational transmission from patients to HCP.
Potentially infectious fluids
The risk for transmission of HIV infection from these fluids is unknown; the potential risk to HCP from occupational exposures has not been assessed by epidemiologic studies in health-care settings.
- cerebrospinal fluid
- synovial fluid
- pleural fluid
- peritoneal fluid
- pericardial fluid
- amniotic fluid.
The following fluids are not considered potentially infectious unless they are visibly bloody; the risk for transmission of HIV infection from these fluids and materials is low.
- nasal secretions
Any direct contact (i.e., contact without barrier protection) to concentrated virus in a research laboratory or production facility requires clinical evaluation. For human bites, clinical evaluation must include the possibility that both the person bitten and the person who inflicted the bite were exposed to bloodborne pathogens. Transmission of HIV infection by this route has been reported rarely.
Per-contact risk of HIV transmission:
- Unprotected anal-receptive sex with a known seropositive partner: 0.82 percent
- Unprotected anal-receptive sex with a partner of unknown serostatus, 0.27%
- Unprotected insertive anal sex: 0.06%.
- Receptive oral sex: 0.04%.
"The overall, unadjusted probability of HIV-1 transmission per coital act is 0·0011 in this Ugandan population, and greater infectivity of predominant HIV-1 viral subtypes is unlikely to account for the explosive HIV-1 epidemic in sub-Saharan Africa. Transmission probability per act varies greatly with the HIV-1 viral load of the HIV-1-infected partner, which suggests that interventions to reduce viral load could reduce transmission.15,28 Younger age and genital ulceration also increased the probability of transmission per act."
Symptoms and complications
Many people who are infected with HIV do not have symptoms for many years. Someone can look and feel healthy but can still be infected. In fact, one quarter of the HIV-infected persons in the United States do not know that they are infected.
The following may be warning signs of advanced HIV infection:
- rapid weight loss
- dry cough
- recurring fever or profuse night sweats
- profound and unexplained fatigue
- swollen lymph glands in the armpits, groin, or neck
- diarrhea that lasts for more than a week
- white spots or unusual blemishes on the tongue, in the mouth, or in the throat
- red, brown, pink, or purplish blotches on or under the skin or inside the mouth, nose, or eyelids
- memory loss, depression, and other neurological disorders
However, each of these symptoms can be related to other illnesses. The only way to determine whether a person's infection status is by testing for HIV infection.
Asymptomatic pericardial effusions are common, especially in advanced stages of immunodeficiency. Less commonly, the effusions are symptomatic or the patient has signs of pericarditis. There are many causes of pericardial effusions in HIV patients.
Pancreatic enzymes may help if fat malabsorption is present.
HIV Wasting Syndrome
Once HIV enters the body, the body starts to produce antibodies — substances the immune system creates after infection. Most HIV tests look for these antibodies rather than the virus itself. There are many different kinds of HIV tests, including rapid tests and home test kits.
- http://www.merckmedicus.com/ppdocs/us/hcp/content/merck/hiv/hivaids/aidsdefi.htmAIDS is defined
- Sepkowitz KA (2001). "AIDS--the first 20 years.". N Engl J Med 344 (23): 1764-72. PMID 11396444.
- Centers for Disease Control (CDC) (1981). "Pneumocystis pneumonia--Los Angeles.". MMWR Morb Mortal Wkly Rep 30 (21): 250-2. PMID 6265753.
- Altman, L.K. (May 11, 1982)"New homosexual disorder worries officials". New York Times
- Barré-Sinoussi F, Chermann JC, Rey F, Nugeyre MT, Chamaret S, Gruest J et al. (1983). "Isolation of a T-lymphotropic retrovirus from a patient at risk for acquired immune deficiency syndrome (AIDS).". Science 220 (4599): 868-71. PMID 6189183.
- Centers for Disease Control (CDC) (1985). "Testing donors of organs, tissues, and semen for antibody to human T-lymphotropic virus type III/lymphadenopathy-associated virus.". MMWR Morb Mortal Wkly Rep 34 (20): 294. PMID 2987657.
- 019655 Drugs@FDA. U S Food and Drug Administration
- Where did HIV come from?. Retrieved on 2007-10-18.
- that is, sex without using condoms
- The Lancet, March 30th 2007
- AEGiS-AIDSWeekly: Per-Contact Risk of HIV: Odds Don't Tell Whole Story - August 9, 1999. Retrieved on 2007-10-18.
- Gray RH, Wawer MJ, Brookmeyer R, et al (2001). "Probability of HIV-1 transmission per coital act in monogamous, heterosexual, HIV-1-discordant couples in Rakai, Uganda". Lancet 357 (9263): 1149–53. PMID 11323041.
- Silva-Cardoso J, Moura B, Martins L, Mota-Miranda A, Rocha-Gonçalves F, Lecour H (1999). "Pericardial involvement in human immunodeficiency virus infection". Chest 115 (2): 418–22. PMID 10027441.
- Moreno R, Villacastín JP, Bueno H, et al (1997). "Clinical and echocardiographic findings in HIV patients with pericardial effusion". Cardiology 88 (5): 397–400. PMID 9286499.
- Cárcamo C, Hooton T, Wener MH, Weiss NS, Gilman R, Arevalo J et al. (2005). "Etiologies and manifestations of persistent diarrhea in adults with HIV-1 infection: a case-control study in Lima, Peru.". J Infect Dis 191 (1): 11-9. DOI:10.1086/426508. PMID 15592997. Research Blogging.
- (1996) "American Gastroenterological Association medical position statement: guidelines for the management of malnutrition and cachexia, chronic diarrhea, and hepatobiliary disease in patients with human immunodeficiency virus infection.". Gastroenterology 111 (6): 1722-3. PMID 8942755.
- Kearney DJ, Steuerwald M, Koch J, Cello JP (1999). "A prospective study of endoscopy in HIV-associated diarrhea.". Am J Gastroenterol 94 (3): 596-602. DOI:10.1111/j.1572-0241.1999.00920.x. PMID 10086637. Research Blogging.
- Nwachukwu CE, Okebe JU (2008). "Antimotility agents for chronic diarrhoea in people with HIV/AIDS.". Cochrane Database Syst Rev (4): CD005644. DOI:10.1002/14651858.CD005644.pub2. PMID 18843696. Research Blogging.
- Anukam KC, Osazuwa EO, Osadolor HB, Bruce AW, Reid G (2008). "Yogurt containing probiotic Lactobacillus rhamnosus GR-1 and L. reuteri RC-14 helps resolve moderate diarrhea and increases CD4 count in HIV/AIDS patients.". J Clin Gastroenterol 42 (3): 239-43. DOI:10.1097/MCG.0b013e31802c7465. PMID 18223503. Research Blogging.
- Carroccio A, Guarino A, Zuin G, Verghi F, Berni Canani R, Fontana M et al. (2001). "Efficacy of oral pancreatic enzyme therapy for the treatment of fat malabsorption in HIV-infected patients.". Aliment Pharmacol Ther 15 (10): 1619-25. PMID 11564002.
- Bobat R, Coovadia H, Stephen C, Naidoo KL, McKerrow N, Black RE et al. (2005). "Safety and efficacy of zinc supplementation for children with HIV-1 infection in South Africa: a randomised double-blind placebo-controlled trial.". Lancet 366 (9500): 1862-7. DOI:10.1016/S0140-6736(05)67756-2. PMID 16310552. Research Blogging.
- Cárcamo C, Hooton T, Weiss NS, Gilman R, Wener MH, Chavez V et al. (2006). "Randomized controlled trial of zinc supplementation for persistent diarrhea in adults with HIV-1 infection.". J Acquir Immune Defic Syndr 43 (2): 197-201. DOI:10.1097/01.qai.0000242446.44285.b5. PMID 16940855. Research Blogging.
- Kelly P, Musonda R, Kafwembe E, Kaetano L, Keane E, Farthing M (1999). "Micronutrient supplementation in the AIDS diarrhoea-wasting syndrome in Zambia: a randomized controlled trial.". AIDS 13 (4): 495-500. PMID 10197378.