One popular hypothesis, proposed in 1989, is that the relatively hygienic environments of affluent societies do not give immune cells enough practice in facing their “real” enemies from the microbial world. They grow up, in other words, soft and pampered. But today there is increasing acknowledgment that the link between lack of childhood exposure is not one of cause and effect. One possibility is that highly hygienic environments may simply allow more children to live long enough to develop an autoimmune disease.
