Many recently emerged pathogens infect the gastrointestinal tract. For example, there are three gastrointestinal protozoal infections which must be reported to the Center for Disease Control. They are Giardia, Cyclospora and Cryptosporidium, and none of them were known to be significant pathogens in the 1970s.
Figure 1 shows the prevalence of gastrointestinal protozoa in studies from the United States and Canada. The most prevalent protozoa in these studies are considered emerging infectious diseases by some researchers, because a consensus does not yet exist in the medical and public health spheres concerning their importance in the role of human disease. Researchers have suggested that their treatment may be complicated by differing opinions regarding pathogenicity, lack of reliable testing procedures, and lack of reliable treatments. As with newly discovered pathogens before them, researchers are reporting that these organisms may be responsible for illnesses for which no clear cause has been found, such as Irritable Bowel Syndrome.