When our first son developed asthma as a 3-year-old, my husband and I felt pretty much blindsided. We were only a little less shocked when the same thing happened to our second son, at the same age.

The disease turned out to be tenacious, and for years both boys needed inhalers or a nebulizer machine several times a day to prevent asthma attacks that could keep them up half the night, coughing and wheezing.

Both had eczema, too, and the kind of food allergies — to nuts, peanuts and shellfish — that can lead to fatal reactions.

What caused all this? My husband and I were mystified, because neither of us had asthma or life-threatening allergies, nor did our parents or siblings. I do have hay fever and allergies to cats and dogs, but I had always considered my symptoms just a nuisance — not a bad omen for the next generation. My husband isn't allergic to anything.

But we seem to have been caught on a rising tide that no one fully understands. Our sons were born in 1984 and 1987, and we encountered an awful lot of children their ages who had the same illnesses, far more than we remembered from our own generation.

Statistics suggest that something strange was occurring in those years. From 1980 to 2003, the prevalence of asthma in children rose to 5.8 percent from 3.6 percent, an increase of about 60 percent, according to the Centers for Disease Control and Prevention.

Other estimates from the disease centers show an even bigger increase in the asthma rates for younger children: a 160 percent jump in those younger than 5 from 1980 to 1994. But changes in data collection starting in 1997 make it hard to compare the figures before and after that year. More recently, the rates seem to have leveled off in the United States and in other Western countries. In any case, about 20 million people in the United States have asthma today, including at least 6 million children, and 5,000 people a year die from it. Children in the inner cities seem to be especially hard hit, with exposure to cockroaches and diesel fumes suspected as the culprits. But the cause is not known for sure.

Worldwide, the disease has also increased. From 1985 to 2001, the prevalence rose 100 percent. About 300 million people have asthma, 255,000 die from it, and deaths could increase by 20 percent over the next 10 years, according to the World Health Organization. The problem is especially severe in developing countries, which are least able to provide the long-term intensive treatment that asthma requires.

Some of the apparent increases may not be real but may have occurred because doctors got better at making the diagnosis. But increased reporting seems unlikely to account for all the new cases. Theories come and go, and when you come right down to it, no one really knows why some people develop asthma and others don't.