Do more infections mean less asthma?

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.