No one baby-sits for 2-year-old Grace King.

At birthday parties, the sociable girl sometimes leaves before cake is served. That's because Grace is allergic to both milk and eggs, and cake or ice cream could send her to the hospital, says her mother, Lauren King, of Boynton Beach.

A well-meaning relative once forgot and gave Grace a bottle of milk. The toddler drank it all, and then started vomiting for hours. It was two weeks before she could eat again, with a doctor's help. Her stomach was so touchy, it has taken six months for her to regain the lost weight.

"I would love for her to go to school a couple of days a week," says King. "But it happens so quickly. If she picks up another kid's snack or drink, her throat closes up and she's going to the hospital."

One-third to one-half of the population has some sort of allergy, and the severity of allergic reactions appears to be growing fast, especially in children. The number of children who react to peanuts tripled between 1989 and 2002, according to a recent European study, which found 3 percent allergic to the legume. The same trend is emerging for many other allergies, including tree nuts, shellfish, dust mites, pollen, mold, insect venom, latex, penicillin and other drugs.

Fort Lauderdale veterinarian Al Brunz faced losing the occupation he loved as wheezing episodes filled his days. A visit with an allergist showed him what he suspected. "Animals are my No. 1 allergy -- I'm violently allergic to cats," he said.

The quest to understand the cause of all this sneezing, coughing and gasping has taken scientists from farm fields to inner cities, from laboratories to grocery stores. The allergic response remains full of paradox: Filth can cause disease and asthma attacks, of course. Yet new studies suggest clean living probably unleashes allergies.

While dogs and cats can make people sneeze, their germy presence seems to prevent allergies in babies. Meanwhile, therapies that short-circuit the allergic response work by exposing people to more of what irritates them.

To understand the puzzle, picture the body's foreign visitors -- bacteria, toxins, pollen, proteins -- encountering specialized cells that memorize their appearance and decide "friend or foe." If the cells spot these visitors again and label them "foe," they bring in assistants who launch a reaction to destroy them.

Several types of destruction sequences can be launched. In nonallergic people, these missions go on without notice, through a process allergists call the Th1 response, referring to the actions of specialized white blood cells called helper-T cells. In allergic people, Th2 cells take the lead. They involve groups of antibodies known as "IgE." These cells probably evolved to fight off intestinal parasites and worms. In the absence of such bugs, they seem to cause allergies.

Grace's first allergic attack happened when she was just 9 months old. The day started with a checkup and booster shots at the pediatrician's. After discussing new foods with the doctor, King decided that evening to introduce her daughter to a yogurt-and-juice drink. Grace seemed to like it. But 15 minutes later, Grace began gasping and crying in a strange way. Then she started vomiting. Her father raced her to the nearby emergency room, where a triage nurse waved him through, and into the arms of a doctor. Grace, they were told, had a food allergy. "It was nightmarish for a while," says King.

The dirt on allergies

Grace's parents can only guess at the cause of her allergy. Was it genes from his family? Hers? Did the booster shots do something? The introduction of a new type of food (yogurt)? They may never know. But they are not alone.

The incidence of allergies is rising fast, and scientists use words like "epidemic" to describe the growth. Eight percent of children 6 or younger have some type of food allergy today, the National Institutes of Health reports. Only 1 percent to 2 percent of adults are affected. About 100 Americans, usually children, die from systemic allergic reactions each year.

Other allergic conditions are on the rise: Eczema -- itchy, allergic skin rash -- is the most common skin condition in children younger than 11. Incidence has increased from 3 percent of children in the 1960s to 10 percent in the 1990s, the NIH says.

"It's not just allergy -- all immune-based diseases have skyrocketed," said Dr. Marc Rothenberg, chief of Allergy and Clinical Immunology for the Cincinnati Children's Hospital.

It's hard to imagine that the Kings' clean house or suburban lifestyle could lead to Grace's severe food allergy, but a concept known as the "hygiene hypothesis" has gained converts.

The thinking is: Clean food and water and indoor living have deprived us of the constant parade of germs and parasites that our great-grandparents survived. As a result, our immune systems never get trained properly. Rothenberg refers to it as the immune system's "delinquency problem."

"The immune system has too much free time, and it is getting into things that it shouldn't," he said.

That theory gained support from a recent New England Journal of Medicine study. Trying to understand why farm children were less likely to have allergies, the researchers looked at germs. They found that farm children's mattresses were covered with bacteria endotoxin, the kind found in farm animal manure. Day and night, those children were breathing, touching, eating and drinking germs, and they appeared to have healthier immune systems as a result, wrote Dr. Scott Weiss of the Channing Laboratory at Harvard University.

Other theories abound. A French scientist showed that as measles and whooping cough have diminished, allergy, asthma and autoimmune diseases have risen. Childhood vaccines may play a role, he noted, writing in the New England Journal of Medicine. At issue may be the type of immune response mobilized by the shots, and its effect on the evolution of an infant's immunity.

Many questions remain. He also raised the possibility that babies who take antibiotics could have problems, because of the way the drugs alter intestinal flora, a key component of the immune system. Another study found that women with allergies who were given large doses of harmless lactobacillis, commonly used as yogurt culture, during their pregnancy, had newborns with significantly less eczema. The incorporation of such harmless microbes into infants' food also helped moderate infants' eczema.

Other research has suggested that environmental pollutants may act like lighter fluid on a grill. They don't cause the allergy fire, but they can make it burn hotter and brighter. Truck exhaust is one powerful sensitizing pollutant. Secondhand cigarette smoke, a flame-retarding chemical used in computer monitors, and even emissions from disposable diapers have shown some ability to heighten the sensitivity of people to allergens in laboratory studies, according to Reuters Health.

That rang true with Fort Lauderdale vet Brunz, who loves jogging almost as much as animals. He found that running through downtown Fort Lauderdale, with all of its traffic and trucks, set off his wheezing. He suspects truck and car exhaust makes his animal allergies worse.

Attention is also turning to Western diets. Weiss, at Harvard, is now studying the role of fats and oils in pregnant women's diets. Some suspect that omega-3 fatty acids, those found in seafood, may help, while trans fats, those partially hydrogenated oils like margarine and shortening, widely used by fast-food makers, including Kentucky Fried Chicken and Burger King, might worsen allergies and asthma.

An allergy-free life

Heading to the allergist for a monthly allergy shot may seem like an extreme step, but evidence is mounting that it works well for treating inhaled allergies and insect-venom allergies, and can prevent new allergies and asthma, especially in children. For Brunz, allergy shots have freed him to return to the things he loves -- jogging, yard work and, most of all, the animals. Once a month, he takes a shot of dust mites in one arm, a shot of dog and cat particles in the other.

Before the shots, he had to reach for an asthma inhaler 10 times a day. Today, one hit of a milder drug suffices. "I've not had a single asthmatic attack since I started immunotherapy," Brunz said.

Immunotherapy -- allergy shots -- works by exposing an allergic person to progressively greater doses of the substance they're allergic to, over a period of years. Eventually, the immune response shifts from the hypersensitive Th2 type to the Th1 type.

That's basically a cure, for many people, said Fort Lauderdale allergist Dr. Linda Cox. "Medications work, but when you stop the meds, the allergies come back," she said. "Immunotherapy is the only intervention that can modify the allergic disease."

Children may benefit most. Two recent studies have shown that the shots prevented allergies from growing into asthma in a significant percentage of allergic children. The shots also prevented children in the studies from acquiring new allergies.

"If you can do something that will turn the process off, it's worth the time investment," said Cox, who is vice chairman of the American Academy of Asthma, Allergy and Immunology's committee on immunotherapy.

Brunz agreed. "I love to be outside, and now I can. I feel extremely fortunate. I wish I had done it years ago."

Other preventive strategies have been scrutinized. The American Academy of Asthma, Allergy and Immunology recommends that parents with allergies introduce new foods to their infants very slowly. Breast-feeding exclusively for the first six months is preferred, the group recommends. Other research has shown that attendance at day care can lower the risk of allergies, as can the presence of multiple pets in the home. Both probably cause a child to encounter more germs than they would otherwise.

For Grace King, the best approach now is to avoid the offending foods, and to keep a loaded syringe of epinephrine on hand at all times, in case of emergency. For some people, childhood food allergies subside on their own, but that's unlikely to happen for Grace. An allergist recently measured the intensity of her reaction to milk protein, to assess her chance for remission.

Grace's mother said the news was sobering. Grace's antibody response was off the charts.

"She's not one of the ones who is going to grow out of it," she said.

Stacey Singer is a freelance writer.