Human intestinal protozoan infections are found worldwide in both developing countries and the industrialized world. Pathogenic intestinal protozoa produce disease by infecting the small or large intestine, or both. Intestinal protozoa are found in highest prevalence in developing countries, where they are responsible for a substantial burden of disease. The small intestinal protozoa Giardia lamblia and Cryptosporidium parvum have their major impact in children, while the large bowel pathogen Entamoeba histolytica infects all age groups but has its most profound effects in adults. The epidemiology of intestinal protozoan infection has developed rapidly with the increase in acquired immunodeficiency states, particularly that relating to cytotoxic chemotherapy for cancer and human immunodeficiency virus (HIV) infection and the acquired immune deficiency syndrome (AIDS). Interestingly, some of the protozoa, particularly …