Parasites differ in their pathogenicity and in their capacity to produce invasive or systemic disease. The subclass coccidia, for example, includes both Toxoplasma and Isospora. However, Isospora is unable to invade the intestinal mucosa and thus produces only enterocolitis, whereas Toxoplasma crosses the intestine and produces severe systemic illness.

Sometimes various forms of the parasite differ in their ability to cause illness. The adult form of Trichinella spiralis remains in the intestine, while the larval form crosses and migrates to striated and cardiac muscle. Amebiasis can result in both intestinal and visceral infections. Pathogenicity may vary among strains within a genus. Infection with Entamoeba can result in an asymptomatic cyst carrier state or hepatic abscesses, depending on the strain involved.

Finally, organisms differ in their virulence. The infectious dose of Giardia and Cryptosporidium is on the order of 101 to 103 organisms. By contrast, the infectious dose of Vibrio cholerae and Salmonella is 105 to 108 organisms. This may be an important factor in the genesis of outbreaks in institutions and day-care centers.

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