Some scientists think that children who live too cleanly when they're young are more likely to develop allergies, because their immune systems don't have a chance to develop properly. It seems to be true that children who are exposed to lots of microbes are less likely to develop allergies, but after a study in Denmark, it isn't as clear whether kids who get infectious diseases are also protected from allergies—in this case, allergic skin reactions.

What the researchers wanted to know: Are children who get infectious diseases passed on from their siblings less likely to have itchy, allergic rashes?

What they did: The Danish are masters of gigantic epidemiological studies. From 1997 to 2002, every pregnant woman in the country who spoke Danish and intended to complete her pregnancy was invited to join the study when she first talked with her doctor about the pregnancy. The mothers took part in a series of four interviews. Partway through the study, questions about itchy rash and eczema were added to the fourth interview. At that time, when children were 18 months old, mothers were asked about rashes and also about infectious diseases (such as colds, diarrhea, and ear infections) their children had. From the interviews, researchers also knew about siblings, pets, and whether the families lived on a farm. Over 24,000 mother-child pairs completed all four interviews.

What they found: Children who spent a lot of time with microbes (for example, children who live around pets or farm animals) were less likely to develop allergic rashes, but the same was not true of children who had infectious diseases such as colds and diarrhea. Those children were more likely to have rashes. Having a large number of siblings protects against allergy, but if this study is right, it's not because siblings pass on infectious diseases.

What this study means to you: This sort of information could help explain why allergies happen, and how to prevent them from developing.

Caveats: Only a few children had diseases like measles that might have a more long-lasting effect on the immune system than colds and diarrhea have.

Find out more: To learn more about the study, go to the Danish National Birth Cohort. Watch a video clip and read about the hygiene hypothesis on PBS's website.

Read the article: Benn, C.S., et al. "Cohort Study of Sibling Effect, Infectious Diseases, and Risk of Atopic Dermatitis During First 18 Months of Life." British Medical Journal. May 22, 2004, Vol. 328, p. 1223.

Free online at: http://bmj.bmjjournals.com