Schematic life cycle of the most recurrent Cryptosporidium coinfective and coemerging parasitic agents. Panel (a). Giardia intestinalis (also called Giardia lamblia or Giardia duodenalis) is a flagellate parasite (Diplomonadida). Both cysts and trophozoites can be found in nondiarrheal feces (diagnostic and infective stages, (1)). Cysts are resistant forms and are responsible for transmission of giardiasis (infective stage, (2)). Infection occurs by the ingestion of water or food contaminated by cysts, or by the fecal-oral route (hands or fomites). In the small intestine, excystation releases trophozoites which multiply by longitudinal binary fission, remaining in the lumen of the proximal small bowel where they can be free or attached to the mucosa. Encystation occurs as the parasites transit toward the colon. Panel (b). Cyclospora cayetanensis is a coccidian parasite (Apicomplexa). When passed in stools, the oocyst is not infective (on the contrary of Cryptosporidium, thus direct fecal-oral transmission cannot occur). In the environment, sporulation occurs after days or weeks, resulting in division of the sporont into two sporocysts, each containing two sporozoites (diagnostic and infective stages, (1)). Fresh food and water can serve as vehicles for transmission and the sporulated oocysts are ingested (infective stage, (2)). The oocysts excyst in the gastrointestinal tract, freeing the sporozoites which invade the epithelial cells of the small intestine. Inside the cells they undergo asexual multiplication and sexual development to mature into oocysts, which will be shed in stools. Panel (c). Blastocystis hominis is a Heterokontid Chromista (Stramenopiles). The thick-walled cyst present in the stools (diagnostic stage, (1)), which varies tremendously in size from 6 to 40 μm, is believed to be responsible for external transmission, possibly by the fecal-oral route through ingestion of contaminated water or food (infective stage, (2)). The cysts infect epithelial cells of the digestive tract and multiply asexually. Vacuolar forms of the parasite give origin to multi vacuolar and ameboid forms. The multivacuolar form develops into a precyst that gives origin to a thin-walled cyst, thought to be responsible for autoinfection. The ameboid form gives origin to a precyst, which develops into thick-walled cyst by schizogony. The thick-walled cyst is excreted in feces. Panel (d). Entamoeba histolytica/dispar is an Amoebozoa parasite. Cysts and trophozoites are passed in feces (diagnostic and infective stages, (1)), the first found in formed, whereas the latest in diarrheal stool. Infection by E. histolytica occurs by ingestion of mature cysts in fecally contaminated food, water, or hands. Excystation occurs in the small intestine and trophozoites are released, which migrate to the large intestine and multiply by binary fission to produce cysts, where both stages are passed in the feces. Cysts can survive days to weeks in the external environment and are responsible for transmission (diagnostic and infective stages, (2)). Trophozoites passed in the stool are rapidly destroyed once outside the body, and if ingested would not survive exposure to the gastric environment. In many cases, the trophozoites remain confined to the intestinal lumen (noninvasive infection) of individuals who are asymptomatic carriers, passing cysts in their stool. In some patients the trophozoites invade the intestinal mucosa (intestinal disease), or, through the bloodstream, extraintestinal sites such as the liver, brain, and lungs (extraintestinal disease), with resultant pathologic manifestations. It has been established that the invasive and noninvasive forms represent two separate species, respectively, E. histolytica and E. dispar. These two species are morphologically indistinguishable unless E. histolytica is observed with ingested red blood cells (erythrophagocytosis). Infective and diagnostic stages, as well as body organs, are graphically reported, when surely assessed in the life cycle of the parasites. Modified from pictures available at the CDC site for parasite identification and diagnosis (http://www.dpd.cdc.gov/dpdx/HTML/Para_Health.htm). Putignani and Menchella, 2010.