Some scientists have proposed that the human immune system needs something to keep it busy in early childhood. Without enough microbes to fight, they say, idle or improperly educated immune cells may make mischief later by inducing reactions against substances that don't merit the effort. This still controversial notion, called the hygiene hypothesis, holds that such children are prone to develop hypersensitivity later—in the form of allergies or asthma (SN: 8/14/99, p. 108: http://www.sciencenews.org/sn_arc99/8_14_99/bob2.htm).
The most recent evidence in this debate shows that young infants kept out of day care and having no more than one older sibling are significantly more likely to develop asthma than are babies who have greater exposure to other children.
Starting in the early 1980s, epidemiologist Anne L. Wright of the University of Arizona College of Medicine in Tucson and her colleagues tracked 1,035 infants from birth. Parents brought the children in for