 From this, he hypothesized that a lack of exposure to "infections" in early childhood transmitted by contact with older siblings could be a cause of the rapid rise in atopic disorders over the last 30 to 40 years.Strachan further proposed that the reason why this exposure no longer occurs is not only because of the trend towards smaller families, but also "improved household amenities and higher standards of personal cleanliness".Although there is substantial evidence that some microbial exposures in early childhood can in some way protect against allergies, there is no evidence that humans need exposure to harmful microbes (infection) or that it is necessary to suffer a clinical infection.