FROM VEGETABLE soup to cheesecake, America leaves its food on the counter to thaw, to cool and to store.

"I thawed a pork roast at room temperature and never got around to cooking it. Can I cook it now? Will it hurt me?"

"I left my cheesecake on the counter to cool overnight. Is it safe to eat?"

"There are a lot of people who call and want us to tell them that what they're doing is right," says Anne Prince, a Fairfax County extension agent and one of the experts to whom people turn when they wonder if eating certain foods will cause illness. Extension specialists, microbiologists and health officials usually don't comply with these wishes, however. "When in doubt," they say, "throw it out."

Nobody wants to tell someone with an improperly stored pork roast to throw $12 into the garbage and chalk it up to experience. But that's what has to be done, says Dr. Robert Gravani, a microbiologist and assistant professor of food science at Cornell University. "We may not be able to save your pork roast ," he says, "but we'll give you the information to prevent the problem the next time."

The way to avoid both food poisoning and food waste is to anticipate the causes and conditions of the poisoning. Knowing how and where the organisms grow makes decisions easier. Cheesecake that has cooled on the counter overnight may be free of contamination--acid in the cream cheese, sour cream and lemon juice discourages bacterial growth. However, if there are lots of eggs and the cake is flavored with lemon rind rather than juice, it could cause illness. And if it's coconut custard (eggs, sugar and milk), it should be thrown out without a second thought.

"The common-sense approach, although it's boring, is time-temperature," says Dr. James Oblinger, a microbiologist at the University of Florida. The longer food remains at temperatures suitable for bacterial growth, the greater the chances of getting sick from the food.

Food should be held at temperatures below 45 degrees or above 160 degrees. Set the stuffed turkey on the table and the clock begins ticking -- most health departments allow two to three hours before declaring the food unsuitable for eating.

Bacterial food poisoning isn't easy to spot. For the most part, the bacteria and their byproducts are odorless and invisible. And contamination isn't easy to predict because most of the bacteria that cause poisoning are all over the place -- human skin, raw meats, eggs, soil and air -- hence the blanket advice of getting rid of any food that might be questionable. (The most peculiar case of food poisoning showed up recently when people became ill from eating watermelon -- an unlikely breeding ground for toxin-producing bacteria.)

The good news, however, is that food poisoning is rarely lethal. "For most normal people," says Oblinger, "the symptoms will resemble the flu." But for the elderly, the very young and those already suffering infection or illness, food poisoning may pose a more serious danger.

Personality profiles of the different bacteria follow, but the moral of the story is invariably the same. The cook should keep hands and utensils clean (don't be afraid to clean the cutting board with diluted chlorine bleach). Moist foods that are high in protein and low in acid should be kept very hot or very cold to prevent bacterial growth.

Foods should be thawed in the refrigerator. If you want to thaw them quickly, do it in cold water to cover.

If you anticipate leftovers, don't allow foods to sit at room temperature for more than two hours, but Gravani advises to get them cooled down and refrigerated as soon as possible. Reheat foods thoroughly and quickly -- use wide, shallow pans rather than deep, narrow ones and put foods into a preheated hot oven rather than one that is warming up.

*W We all hear rumors of botulism, especially during the summer as we put up green beans, corn and other low-acid foods. Botulism, while certainly not the most prevalent food poisoning, is by far the most dangerous -- tiny amounts are lethal.

Clostridium botulinum thrives in an atmosphere with no oxygen -- thus the concern over canned items. The bacteria are sensitive to acid (so they don't grow in most canned tomatoes, fruits and pickles) and to heat (so it's wise to boil canned vegetables and meats for 10 to 15 minutes before consuming).

Botulism is most often a problem with home-canned foods that are underprocessed -- Gravani warns that all recipes be "followed to the letter" when cooks process foods at home. This caution includes following updated canning instructions, not grandmother's 30-year-old recipe for putting up vegetables.

As a rule, however, cooks should take care to process low-acid foods under pressure and should discard any bulging cans, any canned food with an off odor and any food that "spits" when the can is open.

* Only ants and ball park mustard rival the presence of staphylococcus at picnics. Staph poisoning, associated lately with 36,000 pounds of deli-style ham, causes the most infamous food poisoning and is commonly found in creamy, custardy foods and picnic salads such as tuna, chicken and potato.

This organism disproves the when-you-cook-it-you-kill-the-germs theory. While the organism is, indeed, destroyed by heat, the poison lingers to produce what one book graphically describes as "explosive diarrhea," vomiting, cramps and dehydration four to six hours after being consumed. "It's one of the most heat-resistant toxins we know about," says Gravani. Except in extreme cases (usually the very young or very old), it won't kill you--you'll only wish you were dead.

Many people think that spoiled mayonnaise causes the food poison. Not true. The high acid content of mayonnaise actually discourages staph. When mixed with other ingredients, however, the resulting dish is usually lower in acid and the bacteria thrive.

Improper handling procedures increase the chances of staph food poisoning. The bacteria are common on human skin and around the face. If the hands aren't clean or the cook touches his face a lot during cooking, contamination will follow.

* Moving from picnics to parties, we find Clostridium perfringens lurking in sauces, gravies and meats. It, too, causes diarrhea and cramps four to 22 hours after consumption.

A perfect place to encounter this bug is at large, informal dinners. If enthusiastic amateur cooks preparing beef stew for a church dinner, for instance, treat the food improperly, food poisoning could result. The bacteria are heat resistant, and some may remain even after cooking.

Gravani talks of studies showing that large pots full of stew take up to six days in the refrigerator to cool to 40 degrees. This gives the bacteria plenty of time to multiply. When cooking large quantities -- at home or in church or school kitchens -- Gravani advises pouring the stew, spaghetti sauce or chili into large, shallow containers that expose more food to cooler temperatures.

* While salmonella poisoning in the last year has been associated with prepared roast beef recalls from some supermarket deli sections, it is most prevalent in poultry. Naturally, salmonella enjoys the cozy atmosphere of a warm Thanksgiving dinner as much as the rest of us.

Salmonella contamination, which comes from improper handling of food and poor sanitary conditions, is the most common type reported to the Center for Disease Control in Atlanta.

High heat does destroy this germ. Turkey stuffing causes some concern because cooks often use their hands to prepare it and to stuff it into the bird. If heat fails to penetrate the stuffing thoroughly, it becomes a thriving salmonella culture.

Diarrhea, vomiting and cramps usually occur several hours later and last two or three days (and possibly up to seven). The cook diminishes the chances of contamination by washing his hands frequently and using clean utensils to handle the food.

Cross contamination, always a problem, is easy at holiday time because there's so much food being prepared. The salmonella on the turkey may be killed by oven heat, but if other foods -- salads and other dishes eaten cold or barely warmed -- are prepared on the same counter or cutting board, these items may end up causing food poisoning. Even traditional foods meant to cheer revelers -- such as eggnog -- can become culprits in very unpleasant holidays.