Officials in Houston are just beginning to grapple with the health and environmental risks that lurk in the waters dumped by Hurricane Harvey, a stew of toxic chemicals, sewage, debris and waste that still floods much of the city.

Flooded sewers are stoking fears of cholera, typhoid and other infectious diseases. Runoff from the city’s sprawling petroleum and chemicals complex contains any number of hazardous compounds. Lead, arsenic and other toxic and carcinogenic elements may be leaching from some two dozen Superfund sites in the Houston area.

Porfirio Villarreal, a spokesman for the Houston Health Department, said the hazards of the water enveloping the city were self-evident.

“There’s no need to test it,” he said. “It’s contaminated. There’s millions of contaminants.”

He said health officials were urging people to stay out of the water if they could, although it is already too late for tens of thousands.

“We’re telling people to avoid the floodwater as much as possible. Don’t let your children play in it. And if you do touch it, wash it off,” Mr. Villarreal said. “Remember, this is going to go on for weeks.”

Flooding always brings the danger of contamination and disease, though epidemics from floods in the United States have been rare. This inundation, which put nearly 30 percent of the nation’s fourth-largest city underwater, will pose enormous problems, both immediately and when the waters finally recede.