The intestinal protozoan parasites Giardia lamblia, Cryptosporidium parvum/hominis, and Entamoeba histolytica are part of a larger group known as “Category B Priority Pathogens”.  Categorized in 2002 by the National Institute of Allergy and Infectious Disease’s Blue Ribbon Panel on “Biodefense and Its Implications for Biomedical Research”, Giardia, Cryptosporidium and E. histolytica are considered a threat due to their possible dissemination through compromised food and water supplies in the United States (  The Blue Ribbon Panel placed an emphasis on the development and implementation of diagnostic tests and therapies specific for these pathogens.  The three parasites are significant pathogens in the United States and share several common traits that would readily permit spreading of an infectious dose to susceptible populations.

Organism Rate per 100,000 in the U.S. Chlorine Resistance Infectious Dose Environmental Stability
Giardia 9.5 Moderate Low (<100) High
Cryptosporidium 1.4 High Low (<100) High
E. histolytica 1.2 Moderate Low (<100) High