Protozoa are one-celled organisms that are capable of carrying out most of the same physiological functions as multicellular organisms by using highly developed organelles within their cell. Many of the over 45,000 species of known protozoa are parasitic. As parasites of humans, this group of organisms has historically been the cause of more suffering and death than any other category of disease causing organisms.
Intestinal protozoa are common throughout the world and particularly in areas where food and water sources are subject to contamination from animal and human waste. Typically, protozoa that infect their host through water or food do so while in an inactive state, called a cyst, where they have encased themselves in a protective outer membrane and are released through the digestive tract of a previous host. Once inside the host, they develop into a mature form that feeds and reproduces.
Amebic dysentery is one of the more common diseases that often afflicts travelers who visit tropical and sub-tropical regions. This condition, characterized by diarrhea, vomiting and weakness, is caused by a protozoan known as Entamoeba histolytica.
Another protozoan that causes severe diarrhea, but is also found in more temperate regions, is Giardia lamblia. Among Leeuwenhoek's discoveries was G. lamblia, which is a now well-publicized parasite that can infect hikers who drink untreated water in the back country.
Other types of parasitic protozoa infect the blood or tissues of their hosts. These protozoa are typically transmitted through another organism, called a vector, which carries the parasite before it enters the final host. Often the vector is an invertebrate, such as an insect, that itself feeds on the host and passes the protozoan on through the bite wound. Some of the most infamous of these protozoa are members of the genera Plasmodium, that cause malaria; Trypanosoma, that cause African sleeping sickness; and Leishmania, which leads to a number of debilitating and disfiguring diseases.