Endotoxin is infamous for its ability to exacerbate existing allergy and asthma symptoms. Current research supports this phenomenon, demonstrating its significance in the home, as well as in the workplace. At the same time, evidence is emerging that exposure to endotoxin may drive immune development away from the T-helper lymphocyte type 2-mediated allergy and asthma profile. This fits in nicely with the 'hygiene hypothesis', which attributes the past century's rise in allergy and asthma to a reduction in microbial burden. Indeed, infections have been associated with less atopy and asthma. Recent investigations have suggested that naturally-occurring non-infectious exposure to microbial components such as endotoxin might mitigate atopy and asthma as well.