Although children in day care seem to get sick more often than other children do, that is not necessarily a bad thing, researchers suggest. These colds and other infections may be giving their immature immune systems a health workout, resulting in a lower incidence of asthma.

Over the past 20 years, the prevalence of asthma has steadily climbed in the US and worldwide.

The airway inflammation of asthma is often caused by an allergic reaction to an environmental trigger.

Researchers focused on day-care attendance and living with siblings, because these are markers of how likely a child was to have been exposed to infections.

Frequent wheezing was more common among 2-year-olds who had older siblings or had been in day care before the age of 6 months.

But by 6 years of age, the opposite was true, and by age 13, these children were half as likely as other children to have wheezing attacks.

Researchers theorize that when infants are exposed to germs early on, their immune systems are pushed to go in an "infection-fighting direction." Without this push, the immune system's shift to infection fighting is delayed, and it becomes more likely to overreact to allergens -- the dust, mold, and other environmental factors that most people can tolerate.

Researchers also suggest that shrinking family sizes and a growing obsession with disinfecting everything in sight may help explain the rise of asthma over the past couple of decades. One hundred years ago, more people lived on farms, hygiene was poorer, and there were no bacteria-killing drugs. In essence, the modern phenomenon of placing children in sterile environments and giving them antibiotics for every sniffle has helped make them more susceptible to asthma.

The New England Journal of Medicine 2000;343:538-543