The remarkable increase in asthma prevalence that has occurred over the last two decades is thought to be caused by changes in the environment due to improved hygiene and fewer childhood infections. However, the specific infections that limit T helper type 2 (T_H2)-biased inflammation and asthma are not fully known. Infectious organisms, including commensal bacteria in the gastrointestinal tract and hepatitis A virus, may normally induce the development of regulatory T (T_R) cells and protective immunity that limit airway inflammation and promote tolerance to respiratory allergens. In the absence of such infections, T_H2 cells—which are developmentally related to T_R cells—develop instead and coordinate the development of asthmatic inflammation.