The ‘hygiene hypothesis’ takes a number of forms which are not exclusive, but have yet to be articulated as a unifying concept. An inverse relationship between parasite infection and immune disorders was first suggested by Greenwood, who noted the low incidence of rheumatoid arthritis in West Africa
In contrast to the Th2-mediated allergies [mediated through immunoglobulin (Ig)E, mast cells and eosinophils], other modern maladies such as type I diabetes, multiple sclerosis and Crohn’s disease are driven by Th1 responses, or by the more recently defined Th17 cells . Critically, these Th1/17 conditions have been increasing in prevalence in high-income countries as sharply as Th2-dependent allergies. For example, the incidence of type I diabetes is increasing year-on-year at the rate of 3·4% , in tandem with the rise in asthma . Tellingly, the age of diabetes onset has become significantly younger, indicating that this disease is gaining in force within the population. Similar patterns have been observed for multiple sclerosis and Crohn’s disease, and it is difficult to reconcile the accentuation of these diseases with reduced exposure to Th1-stimulating microbes in early life.