Okay, not a parasite. But it is probably the most well known microbial infection of our time. Human Immunodeficiency Virus infects an estimated 35 million people worldwide. There are two main types, HIV-1 and HIV-2, both of which can have sub-types. Subtype B is most common in the Americas and Europe, while Subtype C is the most common in Africa and Asia. HIV-1 is found worldwide, while HIV-2 is endemic to West Africa. They are retroviruses, which are a family of enveloped viruses that replicate in the host cell through a process of reverse transcriptase.
HIV is of zoonotic origin. HIV-1 probably came from Chimpanzees, while HIV-2 came from Sooty mangabeys (another primate). What likely occurred was that a hunter was butchering the primate, which was infected, and their blood entered the hunter, transmitting the disease. Phylogenic evidence indicates that viruses have been transmitted to humans this way on at least 8 occasions.
As most know, HIV is transmitted through sexual contact. Interestingly, heterosexual sex is the most common transmitter worldwide, while homosexual sex is the most common transmitter in the US. However, being the receptive partner in either act can have as high as a 30% risk of transmission. HIV can also be transmitted through IV drug use, from mother to child, from blood transfusion, and from transplanted tissue. Being pricked with a needle that had previously been in contact with HIV positive blood, such as a nurse being accidentally stuck, is only 0.3%. Having other STIs greatly increases the risk of infection.
When the virus first enters the body, the innate immune system does what it would do for any unknown pathogen - it binds the virus to dendritic cells, which then carry it to the lymph nodes. Once here, the virus infects the helper T cells, which were supposed to be recruiting the antibodies to fight it. Because of host immune responses against the cells, after the initial infection the infected person may go into a period of latency, in which they have no symptoms, for as long as 15 years.
When a person is first infected, this viral load is very high. Initial infections often produce flu-like symptoms. This continues for approximately a year after initial infection, but then drops and remains extremely low until a time at which both T cells and anti-HIV antibodies become low in the body. T cells decrease fairly regularly over time in an untreated person, while anti-HIV antibodies remain at a fairly high level until the T-cell count drops too low, at which point they plummet.
When the T-cell count goes down too far, other symptoms begin occurring. This includes unexplained weight loss (over 10% of body weight), recurrent respiratory tract infections, Angular cheilitis, oral ulcerations, fungal infection of nails, and pruritic eruptions on the skin.
Once the host becomes immunocompromised, which means that they no longer have enough T cells to fight off any sort of infection, opportunistic infections start. This means that things which would not normally be able to grow in a healthy person can grow in them, such as Oral Hairy Leukoplaikia, a condition in which the tongue takes on a hairy appearance. Tuberculosis rates are also much higher in patients with HIV.
HIV testing is now much better than it initially was, but it can still take up to 2 weeks for antibodies to appear in the blood at such an extent that a test would show positive, making it very necessary for those doing the test to ask when the most recent day in which a person could have become infected was. Before the new rapid tests were invented, which can give results in under an hour, it is estimated that 90% of people with HIV did not know their status. In one South Africa study done in the 80s, only 17% of people who came to be tested came back for their results. Even now that we have the rapid tests, it is estimated that as many as 50% of people living with HIV do not know their status.
If you'd like to see a visual of HIV infecting host cells, this video is pretty cool: http://www.youtube.com/watch?v=RO8MP3wMvqg
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