The Hygiene Theory, wherein exposure to infectious agents early in life seems to reduce the incidence of allergies,  asthma and  auto-immune diseases,   is very likely valid...but it is not new.   This was first suggested in the 1960's.   Even before that, in the 1940's, Drs.  Robert Cook and  William Sherman,  pioneers in Allergy and Clinical Immunology - and my teachers in the field - defined and used 'Stock Bacterial Vaccine' for immunomodulation of asthmatic and other 'allergic reactions' to infections.  The active component  of this treatment, which I have used for decades, has long been suspected to be its Endotoxin content, a point now being validated by current research.    A related assertion recently popularized in the media, however, requires a note of caution.  The presence of cats and/or dogs in the household of an infant in the first year of life seems to provide the 'Hygiene Theory' benefit; but very likely only during that limited time.   Thereafter, dog dander and cat antigen (mainly in cat saliva - almost impossible to get rid of once introduced)  are some of the most highly sensitizing proteins that affect allergic individuals.  Then try to get rid of the pets!  Better to get the benefit elsewhere; eg.  day care centers, reduced use of antibiotics, etc.

GS