"The prevalence of asthma and allergies has increased dramatically worldwide in recent years," says Anthony S. Fauci, M.D., director of NIAID. "This study provides evidence that diminished exposure to early immunological challenges could be one of the reasons for this trend."

"The hygiene hypothesis is widely recognized but largely unproven," says Kenneth Adams, Ph.D., who oversees asthma research funded by NIAID. "The findings of this study strengthen the hypothesis and, after more research, could lead to preventative therapies for asthma and allergies."

The authors of the study followed the medical records of 835 children from birth to age 1, documenting any fever-related episodes. Fever was defined as a rectal temperature of 101 degrees Fahrenheit or above. At age 6 to 7 years, more than half of the children were evaluated for their sensitivity to common allergens, such as dust mites, ragweed and cats.

Researchers found that, of the children who did not experience a fever during their first year, 50.0 percent showed allergic sensitivity. Of those who had one fever, 46.7 percent became allergy-prone. The children who suffered two or more fevers in their infancy had greater protection, with only 31.3 percent showing allergic sensitivity by ages 6 to 7.

In particular, fever-inducing infections involving the eyes, ears, nose or throat appeared to be associated with a lower risk of developing allergies, compared with similar infections that did not result in fevers.

"We didn't expect fever to relate with such a consistent effect," says Christine C. Johnson, Ph.D, M.P.H., senior research epidemiologist of the Henry Ford Health System in Detroit, MI, and one of the co-authors of the study. "It also was interesting that the more fevers an infant had, the less likely it was that he or she would be sensitive to allergies."

Dr. Johnson says that more research is needed to establish if early fevers have a direct effect on allergic development in children. Additionally, she and the other authors are working to determine if early exposure to pets as well as high levels of bacteria could also lower allergy risk. "If we can uncover which environmental factors affect allergic development and why, it may be possible to immunize children against these conditions," she says.