Protozoa are single-celled eukaryotic organisms in the kingdom Protoctista. Unlike fungi and plants, they have no cell wall, and most are heterotrophic, feeding by engulfing food or absorbing nutrients. Many live freely in water or soil, but some are parasites that cause serious diseases in humans and animals.Pathogenic protozoa often have complex life cycles with several different forms, which lets them survive in different environments and evade the immune system. Many are carried between hosts by a vector, an organism (usually an insect) that transmits a pathogen without being harmed by it. Others spread through contaminated food and water.Important protozoan diseases include:
• malaria, caused by Plasmodium and carried by Anopheles mosquitoes
• sleeping sickness, caused by Trypanosoma brucei and carried by tsetse flies
• leishmaniasis, caused by Leishmania and carried by sandflies
• amoebic dysentery, caused by Entamoeba histolytica and spread through contaminated water and food
• toxoplasmosis, caused by Toxoplasma gondii, often caught from cat faeces or undercooked meat.Many protozoan parasites live inside host cells for part of their life cycle, for example Plasmodium inside liver cells and red blood cells. This hides them from antibodies, making them difficult for the immune system to clear. Because they are eukaryotes, few drugs kill them without side effects, and there are no fully effective vaccines for most protozoan diseases.
• malaria, caused by Plasmodium and carried by Anopheles mosquitoes
• sleeping sickness, caused by Trypanosoma brucei and carried by tsetse flies
• leishmaniasis, caused by Leishmania and carried by sandflies
• amoebic dysentery, caused by Entamoeba histolytica and spread through contaminated water and food
• toxoplasmosis, caused by Toxoplasma gondii, often caught from cat faeces or undercooked meat.Many protozoan parasites live inside host cells for part of their life cycle, for example Plasmodium inside liver cells and red blood cells. This hides them from antibodies, making them difficult for the immune system to clear. Because they are eukaryotes, few drugs kill them without side effects, and there are no fully effective vaccines for most protozoan diseases.