Given the unexplained explosion of asthma and worries that smaller families and households scrubbed with antibacterial cleansers may be weakening people's immune systems, the finding offers important clues to researchers seeking to prevent children from developing the sometimes-fatal respiratory disease.
It also could reassure parents feeling guilty about putting their infants in day care.
The University of Arizona study found that children who attended day care in their first six months or had two or more older siblings were about half as likely to have asthma at age 13 as youngsters who had one or no older siblings and did not attend day care until they were older.
"This paper reflects the growing belief that the more sterile the early environment, the more problems later in life," said Dr. Leonard Bielory, director of the Asthma and Allergy Research Center at the University of Medicine and Dentistry of New Jersey in Newark.
According to the hot new "hygiene theory," children who rarely get outside and get dirty are not being exposed to enough germs to stimulate proper development of their immune systems.
Researchers at the University of Arizona College of Medicine have been following about 1,000 children for 15 years, studying their respiratory health and allergens in their environment. After previously showing that children in day care get more respiratory infections than others, the researchers examined more closely the role of day care and family size.
They found that protection against asthma came from frequent exposure to other youngsters, but only if the contact took place in the first six months, a key period for a new immune system.
The theory is that if the immune system isn't stimulated early in life by germs, it begins overreacting to allergy-inducing substances, said Anne L. Wright, a pediatrics research professor who led the study.
The children most exposed to other youngsters were about 40 percent more likely than the group with less contact to suffer from frequent wheezing in their first few years.
But doctors believe that in toddlers, wheezing usually is due to their small airways or respiratory infections. And wheezing almost always disappears by age 6 unless the child has asthma.
The new study, funded by the National Heart, Lung and Blood Institute, was published Thursday in the New England Journal of Medicine.
Asthma cases jumped 158 percent from 1980 through 1998, with many of the new cases among children, according to the Centers for Disease Control and Prevention.
Asthma, the most common chronic childhood disease, is an incurable condition in which allergic reactions to such things as pollen and dust mites trigger a narrowing of the airways, wheezing and trouble breathing.
Astma afflicts some 17 million Americans, including at least 5 million under 18 and about 210,000 in Arizona, and it kills about 5,400 people annually. While it is partly inherited, less-understood factors also are at work.
Two recent German studies similarly found asthma less common in children who started day care early.
Dr. Marshall Plaut, chief of allergic mechanisms at the National Institute of Allergy and Infectious Diseases, said the Arizona group's study is the strongest evidence of this idea to date.
But he said more research is needed on how children's immune systems mature, how that affects whether they develop asthma and whether the apparent benefits of early infections outweigh the dangers.