Cryptosporidium is another protozoan that can produce serious complications in patients with AIDS. Cryptosporidium was described in the 19th century, but widespread human infection has only recently been recognized. AIDS patients, however, can develop fatal toxoplasmic encephalitis. Toxoplasma gondii, a very common protozoan parasite, usually causes a rather mild initial illness followed by a long-lasting latent infection. However, this parasite produces a frequently fatal pneumonia in immunosuppressed patients such as those with AIDS. Evidence suggests that many healthy persons harbor low numbers of Pneumocystis carinii in their lungs. Many protozoan infections that are inapparent or mild in normal individuals can be life-threatening in immunosuppressed patients, particularly patients with acquired immune deficiency syndrome (AIDS). In geographic areas of high prevalence, well-tolerated infections are often not treated to eradicate the parasite because eradication would lower the individual's immunity to the parasite and result in a high likelihood of reinfection. Individuals whose defenses are able to control but not eliminate a parasitic infection become carriers and constitute a source of infection for others. Protozoan diseases range from very mild to life-threatening. Some species are considered commensals, i.e., normally not harmful, whereas others are pathogens and usually produce disease. Virtually all humans have protozoa living in or on their body at some time, and many persons are infected with one or more species throughout their life. Between 16, he described, in addition to free-living protozoa, several parasitic species from animals, and Giardia lamblia from his own stools. Anton van Leeuwenhoek was the first person to see protozoa, using microscopes he constructed with simple lenses. The fossil record in the form of shells in sedimentary rocks shows that protozoa were present in the Pre-cambrian era. More than 50,000 species have been described, most of which are free-living organisms protozoa are found in almost every possible habitat. The Protozoa are considered to be a subkingdom of the kingdom Protista, although in the classical system they were placed in the kingdom Animalia.
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