Several species are classified as commensals, which implies that they do not normally do any harm to the host, whereas other species are considered pathogens and often cause sickness. Infections caused by protozoa may range from being absolutely innocuous to being lethal. Individuals whose immune systems can keep a parasite infection under control but not totally eliminate it are considered to be carriers of the illness and may transfer it on to other people. As eradication would reduce the individual's immunity to the parasite and result in a significant risk of reinfection, well-tolerated infections that occur in geographic regions with a high frequency are frequently not treated to eliminate the parasite. This is done because of the high frequency.As there are not enough trustworthy treatments, there are not enough immunizations that are successful, and there are other problems as well, such the difficulties of managing vectors, the World Health Organization has picked six ailments as the focus of intensified research and training. Malaria, trypanosomiasis, and leishmaniasis are examples of these sorts of disorders caused by protozoa.
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