Public health threats rise to the surface during a flood like the one in the Midwest with animal waste runoff from farms and overflowing city sewer lines. But people are usually smart enough to avoid what can make them sick, experts say.

For example, the 1993 floods in the same region produced no illnesses from contaminated water despite the worries of floating waste, said Nancy Hall, a public health microbiologist at the University of Iowa's University Hygienic Laboratory in Iowa City.

"Typically we don't see the outbreaks of diseases that people fear," said Mike Allred, of the Centers for Disease Control. His office, which handles emergency response, so far has not been asked to provide help to local public health agencies in the flood region.

One concern from flooding is combined sewer overflows, said environmental health professor Jonathan Patz at the University of Wisconsin. That's when drainage systems designed to collect rainwater runoff and domestic sewage in the same pipe overflow, sending untreated human sewage and industrial waste into rivers and other waters. According to the U.S. Environmental Protection Agency, Iowa has 19 of these types of overflow systems and Illinois has about 100.

There also is potential contamination from farm runoff containing animal waste.

"Any flood water will and does contain sewage and fecal material," Hall said. "Sewage can have a lot of bacteria, viruses and parasites that can make you sick.

"We have to be vigilant," she said. That means listening to local officials about whether the water is safe to drink, boiling it if it is not, and testing individual wells after flooding. And people usually are good about that, Hall said.

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People can get bacterial infections from contact with floodwater or tetanus from injuries amid the disaster, but both can be prevented with good hygiene and up-to-date vaccinations, officials said.

There are just as many problems after floodwaters recede, according to the CDC. Mold in flooded buildings can cause respiratory problems. Mosquitoes can increase, carrying diseases such as West Nile, so public health officials will be monitoring for that. Also, people often have heart problems, accidents, electrocutions and carbon monoxide poisoning from running generators indoors, Allred said.