Several epidemiological studies provide support for the “hygiene hypothesis”, which suggest that a cleaner environment and fewer childhood infections cause the increase in allergic diseases.
The developing immune system apparently requires microbial exposure in early life to stimulate Th1 lymphocytes. The relative absence of microbes thereby favors the development of Th2-driven responses, which is associated with allergic diseases. Recent studies have shown that markers of poor hygiene such as hepatitis A, Helicobacter infection and infection with herpes simplex virus was associated with less asthma, hay fever and allergen sensitization. Similarly, exposure to Mycobacterium tuberculosis or to the environmental mycobacteria were proposed to protect from allergy, introducing the concept that the administration of mycobacteria and their products may therefore be used as vaccines aimed at reducing Th2 responses.