Allergy treatment is a burgeoning sector of the economy. Annually, $3.4 billion is spent on medication and doctor visits. There is plenty of room for growth--only an estimated 12 percent of hay fever sufferers seek medical treatment. New drugs debut almost every year. (Recently, there has been a rush to fill the vacuum left by the antihistamine Seldane, which was yanked from the market after eight people died when Seldane reacted adversely with other drugs.) None of the three major prescription antihistamines (Allegra, Claritin, Zyrtec) seems to be consistently more effective than the others. All relieve symptoms in a vast majority of patients. Unlike the pills, immunotherapy shots attack the underlying problem, not just the symptoms. Each week patients are injected with a small dose of allergens to build their immunity. Studies show the shots cure 80 percent of patients after several years of treatment. But they are a huge hassle and costly--nearly $1,000 a year.

Why hasn't naturalselection corrected the immune system's misguided response? Why do allergies afflict an increasing number of victims if they serve no useful purpose? A few theories:

The Victorious Immune System: Evolutionary biologists speculate that a hypersensitive immune system was needed back when parasites were ubiquitous and deadly. Hay fever was the small cost of survival. And since allergies rarely shorten life spans or discourage mates, it is unlikely natural selection will ever weed them out.

Theory of the Leisure System: Modern medicine and hygiene have licked most of the major problems that used to preoccupy our immune system. Now, with nothing to regularly contend with, this theory argues, the system is set off by the most harmless foreign particle.

Western-Style Living: Here blame goes to the advent of poorly ventilated, densely populated living spaces--allergy traps--and the increased popularity of domestic pets. In addition, there is the onset of pollution, which carries mites.

Hypochondria: More people are allergic today because more people are diagnosed as allergic rather than as suffering from colds. The cynical spin is that pharmaceutical companies have duped the public into believing that they have allergies and need drugs to fix them.