Amoebas – blob-shaped microbes linked to several deadly diseases – contaminate drinking-water systems around the world, according to a new analysis. The study finds that amoebas are appearing often enough in water supplies and even in treated tap water to be considered a potential health risk.

A number of these microorganisms can directly trigger disease, from a blinding corneal infection to a rapidly lethal brain inflammation. But many amoebas possess an equally sinister if less well-recognized alter ego: As Trojan horses, they can carry around harmful bacteria, allowing many types to not only multiply inside amoeba cells but also evade disinfection agents at water-treatment facilities.

Even though recent data indicate that amoebas can harbor many serious waterborne human pathogens, U.S. water systems don’t have to screen for the parasites, according to study coauthor Nicholas Ashbolt of the U.S. Environmental Protection Agency’s National Exposure Research Laboratory in Cincinnati. He coauthored a study of amoebas’ ‘yet unquantified emerging health risk