This article offers a mechanism to explain the hygiene hypothesis featured prominently in past issues of Science News. If exposure to microbes has a beneficial effect on the immune response of mice, it may also help humans as well. The relatively antiseptic environments that many Western children experience today as compared to the past may explain the skyrocketing incidence of diseases like asthma.

Simcha Pollack
Queens, N.Y.

It is true that the hygiene hypothesis—that modern cleanliness throws our immune systems out of balance because of less exposure to microbes early in life—has been used to explain rising asthma and allergy rates. However, it doesn’t apply to the recent study, which focused on intestinal bacteria. There is little reason to think that people have fewer bacteria in their intestines now than they did in the past. —Tina Hesman Saey