RIO DE JANEIRO — Athletes competing in next year’s Summer Olympics here will be swimming and boating in waters so contaminated with human feces that they risk becoming violently ill and unable to compete in the games, an Associated Press investigation has found.

An AP analysis of water quality revealed dangerously high levels of viruses and bacteria from human sewage in Olympic and Paralympic venues — results that alarmed international experts and dismayed competitors training in Rio, some of whom have already fallen ill with fevers, vomiting and diarrhea.

It is the first independent comprehensive testing for both viruses and bacteria at the Olympic sites. Brazilian officials have assured that the water will be safe for the Olympic athletes.

Extreme water pollution is common in Brazil, where the majority of sewage is not treated. Raw waste runs through open-air ditches to streams and rivers that feed the Olympic water sites.

As a result, Olympic athletes are almost certain to come into contact with disease-causing viruses that in some tests measured up to 1.7 million times the level of what would be considered hazardous on a Southern California beach.

Despite decades of official pledges to clean up the mess, the stench of raw sewage still greets travelers touching down at Rio’s international airport. Prime beaches are deserted because the surf is thick with putrid sludge, and periodic die-offs leave the Olympic lake, Rodrigo de Freitas, littered with rotting fish.