"A pediatrician told me I was being silly," says Pamela Liflander, who repeatedly asked about her infant's oozing, blistering rashes, his constant spitting up. "The doctor said no child under three has real allergies, and the rashes and vomiting would go away. But I'd had a child before Cameron. I knew what a healthy baby looked like. This was not a healthy baby."

Cameron continued to look sickly, but his growth rate was on the charts. He drank breast milk for almost a year, and Pamela introduced other foods gradually. One day she gave him a bite of tuna. Cameron turned red, swelled like a sea sponge, and choked. Benadryl took care of it, but with the next anaphylactic reaction they ended up in the emergency room near their Riverside, Connecticut, home. It would be the first of many visits.

Just how many things could one child be allergic to? Over the next few years, the Liflanders would find out.

Suppose that 54.3 percent of U.S. citizens had cancer. That figure might set off a nationwide panic—a search for something wrong with our diet, our environment, our activity levels . . . something. In fact, that's the number of Americans who show a positive skin response to one or more allergens (although not everyone who tests positive has an actual allergic disease such as rhinitis, asthma, or eczema).

The manifestations of allergy—sneezing, wheezing, itching, and rashes—are signs of an immune system running amok, attacking foreign invaders that normally mean no harm. Allergens include pollen, dust mites, mold, food, latex, drugs, stinging insects, or any of the other oddball substances to which the body can choose to react, or overreact.

Asthma is a big contributor to keeping allergists in business. This chronic inflammation that causes airways to constrict affects about 20 million Americans, twice as many as 20 years ago. About 4,000 die each year. But all told, allergies rarely kill. They just make the sufferer miserable—sometimes for brief periods, and sometimes for life.

The U.S. is not the only country with high allergy rates. In the U.K. more than 20 percent of the population has active allergies. New Zealand, Australia, Ireland, and the U.K. have the highest prevalence of asthma in the world. Allergies, like obesity, are essentially an epidemic of modernity. As countries become more industrialized, the percentage of population afflicted tends to grow higher. There are remote areas of South America or Africa, for example, where allergies are virtually nonexistent.

At first glance the problem of allergies seems simple, and for most of us the solution is simple too: a handy drug like Zyrtec or Atrovent to treat the symptoms.