You've probably heard of the hygiene hypothesis: too much cleanliness may disturb your immune system. This hypothesis was introduced in 1989 by Strachan to explain data on hay fever in a sample of 17,414 British children born during one week in March 1958. He performed a statistical analysis of 16 social and environmental factors. The results showed that children with the largest number of older siblings were the least likely to develop allergies including hay fever and eczema (skin rash).
Strachan suggested that viral infections in early childhood may reduce the risk of allergic disease, assuming that older children transmit infections to their younger siblings.
Over the last century, infectious diseases have declined, while allergic and autoimmune illnesses have risen dramatically. Could the hygiene hypothesis explain this general trend? In 2000, Strachan wrote that his hypothesis has held up well. Other recent papers conclude that while the relationship of immune system to environment is complicated, exposure to germs is not always bad.
A team of British researchers performed an extensive literature search to see how much difference kitchen hygiene makes in prevention of diarrheal diseases. The results were surprising: poor hygiene was slightly better. Use of the same cutting board for raw and cooked meats and other food was the factor best linked to better health. Other factors, such as use of antibacterial or disinfectant cleaning products, showed no effect.
This does not mean we should try to be dirtier; the authors note there are great difficulties in doing research on people. A team from the University of California, San Francisco, pointed out similar surprises in the published research on children with asthma. In one study, children who had more upper respiratory illness in the first year of life had less asthma, wheezing, and skin allergies at age 7. Children with lower respiratory infections, however, got more asthma. Other studies found indications of an infectious cause of asthma.