New Orleans residents who return to their soggy homes and businesses this week are in for some rude lessons in physics and biology, experts say.

Lesson one will focus on the surprising amount of damage that floodwaters can wreak very quickly upon wood-frame buildings. Lesson two will be an equally difficult tutorial in the astonishing versatility of mold and other fungi, which are poised to enjoy a major population explosion in the steamy microbial incubator that was once the Big Easy.

Wooden structural supports that have become saturated under water will in many cases be warped and are unlikely to straighten out again as they dry, making structures unsafe, according to engineers with flood experience.

Even those structures deemed sound enough to be salvageable will soon come under a major assault from some of Mother Nature's smallest invaders: Health-threatening molds and wood-rot fungi that are very difficult to control and can consume a house from the bottom up.

Beyond the obvious need to remove carpets and floor pads, owners will have to tear out sheetrock and other kinds of porous wallboards, along with any insulation inside walls, which tends not to dry out if left in place. Wiring will have to be replaced, as will many gas lines and plumbing components.

"They're pretty much going to have to strip a lot of these houses down to the studs," said Jon Heintz of the Applied Technology Council, a nonprofit engineering organization in Redwood City, Calif., that will be training many of the inspectors who will soon be swarming into New Orleans to judge which buildings can be salvaged.

"It's pretty simple," said Paul H. Gilbert of Parsons Brinckerhoff, a Seattle-based global engineering firm. "If the water gets above the foundation for any period of time . . . it will likely be cheaper and safer . . . to clear the site and start fresh."

If there is one silver lining, urban entomologists said, it is that the flood probably took a huge toll on New Orleans' famously robust population of termites, which have been chewing up the city's buildings and trees to the tune of $300 million in damage every year.

Federal officials have said little about their plans for assessing the structural damage left in Hurricane Katrina's wake. But with residents of some neighborhoods getting the go-ahead to return to their homes over the weekend, experts warned of physical and medical risks that residents will face as they begin to dig out their homes.

For homes with concrete foundations, residents will first have to see whether those foundations have cracked or shifted. Floodwaters can easily scour beneath the edges of concrete slabs, causing an irreversible settling that can leave the wooden structure above unsound.

And although basements are unusual in southern Louisiana, where flooding is so common that even the dead are interred above ground, any structure that does have a well-sealed cellar will have to be checked to see if it has been displaced.

"Basements that are watertight can pop up out of the ground and float, like boats, when they're surrounded by water," Heintz said.

Also at risk of irreversible damage are weight-bearing wooden elements, such as posts, beams and floor-supporting joists. When wood fibers absorb water, they expand and weaken. Horizontal pieces tend to sag under the weight of the house and typically do not recover upon drying.

Even after the wood seems to have dried, the slow and destructive process of rot will often continue -- a process by which various fungi consume cellulose, the major ingredient in wood and its major source of structural strength. Some fungi, including the one that causes so-called dry rot, are expert at drawing water from distant parts of a structure even as they colonize otherwise-dry wooden beams, gradually weakening those beams even though they are dry.

That is one reason it is so important to remove all materials that might be holding moisture anywhere in a once-flooded home, including paper-covered gypsum board and insulation between walls. Any good source of moisture can nurture microbial growth at a distance.

The fungi that cause wood rot work slowly and generally do not pose major health hazards. But other fungi can overwhelm a home within days after floodwaters recede, sickening residents to the point of their needing hospitalization, experts said. These are molds, common at low levels in many a U.S. bathroom but able to enjoy massive blooms in warm and recently saturated Gulf Coast homes.

"There are hundreds of types of mold spores floating around all the time, just waiting for a wet surface to land on," said Claudette Reichel, a housing specialist with the Louisiana State University Agricultural Center in Baton Rouge.

The best food for molds is paper, Reichel said, which is one reason they grow so readily on moist, paper-covered sheetrock. Molds also feed easily on pressed-wood products and particleboard.

Mold spores can trigger asthma and allergic reactions in people who are sensitive to them. Even people who are typically tolerant of mold may find themselves becoming ill once they start stirring up massive doses in the cleanup process, Reichel said. She strongly encourages people to wear respirators or masks with ratings of at least "N95," which means the pore size is small enough to capture mold spores.

Diluted bleach kills mold, but even dead spores can cause health problems when inhaled. That means that the risk of serious asthma attacks -- a syndrome especially common among African Americans -- is likely to linger long after New Orleans's floodwaters have ebbed, Reichel said. And because bleach does not leave any mold-fighting residue, surfaces will need to be treated repeatedly over the weeks ahead as new spores settle and try to colonize damp surfaces.

Wooden structures that survived Katrina may find themselves better off in one regard: The city's termites have probably suffered a significant setback.

Termites prefer wet wood to dry but, as air-breathing insects that generally live underground, they do not do well in floods, said Gregg Henderson, an urban entomologist at the LSU AgCenter.

After Hurricane Andrew blew through the Southeast in 1992, Henderson and colleagues measured termite populations in parts of Georgia that had flooded, and compared their findings to tallies that had been done before the rains.

"We showed a 77 percent decrease after the hurricane," Henderson said, "so floods can certainly affect termite numbers."

There is one caveat, Henderson warned: New Orleans is largely populated with an alien species of termite inadvertently imported from East Asia several decades ago. These Formosan termites, it turns out, often build their nests inside walls and in other places above ground.

The big question now, Henderson said, is whether the termites planned better than humans did by building enough of their nests above flood level.