The hygiene hypothesis is the proposal that reduced exposure to microbes in early childhood leaves the immune system poorly calibrated, and that this explains the rise of allergic and autoimmune disease in wealthy countries. It is one of the most influential ideas in modern immunology, it is widely misunderstood, and its original form is no longer accepted by most of the researchers working on it.
David Strachan noted in 1989 that hay fever was less common in children with more older siblings, and suggested that infections transmitted within large families in early childhood might be protective. The broader pattern he was addressing is not in doubt: allergic conditions including asthma, hay fever, eczema, and food allergy rose substantially through the twentieth century in industrialised countries, and remain markedly less common in rural and low-income settings.
Several problems emerged.
The mechanism does not appear to be infection. Children who have more childhood infections do not generally have less allergy, and some infections increase risk. The protective factor appears to be exposure to a diverse environmental microbiota rather than to disease.
The strongest evidence points somewhere more specific. Children raised on traditional farms have markedly lower rates of asthma and allergy, and the effect tracks exposure to livestock and unprocessed milk. A widely cited comparison of Amish and Hutterite communities in the United States, genetically similar and both farming, found asthma several times less common among the Amish, who use traditional methods with animals close to the home, than among the Hutterites, who farm industrially. The difference tracked house dust microbial content rather than infection history.
The name is actively misleading. Nothing in the evidence suggests that handwashing, clean water, or sanitation cause allergy, and public health researchers have objected that the label undermines hygiene messaging that saves lives. Several proposed replacements exist, including the old friends hypothesis and the biodiversity hypothesis, though none has displaced the original name in public use.
The current framing is that the immune system develops in expectation of particular microbial inputs, principally environmental bacteria and the organisms that colonise the gut, and that their absence leaves regulatory mechanisms underdeveloped. The old friends formulation extends this to helminths and other organisms present throughout human evolution and largely eliminated in wealthy countries.
What remains disputed is how much of the rise this explains at all. Antibiotic use, caesarean delivery, formula feeding, diet, vitamin D, air pollution, and reduced time outdoors are all candidate contributors, and disentangling them in observational data is very difficult. Deliberate helminth therapy has been trialled with results that have not been consistent. And the honest position is that a large, real, and consequential epidemiological trend still lacks an agreed explanation.
