The top three parasitic infections in the U.S. are 1) trichomoniasis, a sexually transmitted disease caused by the protozoan trichomoniasis vaginalis, with about 7.4 million cases annually; 2) Giardia, a protozoan that causes intestinal problems, with about 2 million infections annually; and 3) cryptosporidium, a protozoan infection with about 300,000 infections per year.