Epidemiology

Many diseases related to inflammation such as type 1 diabetes, rheumatoid arthritis and allergic diseases—hay fever and asthma—have increased in the Western world over the past 2-3 decades.[70] Rapid increases in allergic asthma and other atopic disorders in industrialized nations probably began in the 1960s and 1970s, with further increases occurring during the 1980s and 1990s,[71] although some suggest that a steady rise in sensitization has been occurring since the 1920s.[72] The incidence of atopy in developing countries has generally remained much lower.[71]

Although genetic factors fundamentally govern susceptibility to atopic disease, increases in atopy have occurred within too short a time frame to be explained by a genetic change in the population, thus pointing to environmental or lifestyle changes.[71] Several hypotheses have been identified to explain this increased prevalence; increased exposure to perennial allergens due to housing changes and increasing time spent indoors, and changes in cleanliness or hygiene that have resulted in the decreased activation of a common immune control mechanism, coupled with dietary changes, obesity and decline in physical exercise.[70] The hygiene hypothesis maintains[83] that high living standards and hygienic conditions exposes children to fewer infections. It is thought that reduced bacterial and viral infections early in life direct the maturing immune system away from TH1 type responses, leading to unrestrained TH2 responses that allow for an increase in allergy.[41][84]

Changes in rates and types of infection alone however, have been unable to explain the observed increase in allergic disease, and recent evidence has focused attention on the importance of the gastrointestinal microbial environment. Evidence has shown that exposure to food and fecal-oral pathogens, such as hepatitis A, Toxoplasma gondii, and Helicobacter pylori (which also tend to be more prevalent in developing countries), can reduce the overall risk of atopy by more than 60%,[85] and an increased prevalence of parasitic infections has been associated with a decreased prevalence of asthma.[86] It is speculated that these infections exert their effect by critically altering TH1/TH2 regulation.[87] Important elements of newer hygiene hypotheses also include exposure to endotoxins, exposure to pets and growing up on a farm.[87]