“If you’re on a well, you are your water system,” said Professor Marc Edwards, a drinking water expert at Virginia Tech. “That is why we have a special outreach effort, a collaboration with Louisiana State University, trying to get information to people who have private wells.”

Can I get H.I.V. from exposure to the flood water?

No.

How’s the air?

Not so good. In most cases the toxic air pollution given off by the refineries and chemical plants during emergency shut downs won’t cause severe problems right away. But older adults, those with asthma and the immune-compromised may develop inflammations and other ailments. Long-term risks won’t be known until health officials figure out what, exactly, has been spewed into the air.

Residents going back to their houses will have to rip out wallboard that has developed mold, which is also a health risk. And be careful when tearing out cabinets or walls: Houses may have asbestos and those built before 1978 are likely to have lead paint. You don’t want to expose yourself or your children.

What’s the risk of mosquito-borne diseases like Zika in the wake of the flooding?

At the moment, the risk is lower than normal. Floods and high winds whisked billions of mosquitoes and their larvae to their doom in the Gulf of Mexico. “When the water is raging and surging, everything’s washed away,” said Mustapha Debboun, chief of mosquito control for Harris County, which includes Houston.

But once the waters settle, there will be countless low spots, old tires, flowerpots and the like where mosquitoes can begin rebuilding their populations.

After Labor Day, Dr. Debboun said, he will send teams out to see how many land on an arm in a minute. If the situation gets bad enough, he said, he will start aerial spraying and probably ask for help from the federal agencies.