The hygiene hypothesis was first described in 1989 by David P. Strachan, a British epidemiologist, who noticed that the more children in a family, the lower the rates of allergies and eczema. Children in large families tend to exchange colds and other infections more often than children with fewer siblings, and this increased exposure to pathogens perhaps protected these children from allergies. That same year, Erika von Mutius, an epidemiologist at Munich University, was looking into the effect of hygiene on asthma. Children from dirtier East Germany, she was shocked to find, had dramatically less asthma than their West German counterparts living in cleaner, more modern circumstances. The East German children had likely been exposed to many more viruses and bacteria. According to the hygiene hypothesis, exposure in early childhood to infectious agents programs the immune system to mount defences against disease-causing viruses, bacteria and parasites. Better sanitary conditions deprive the immune system of this training, so the body fights against harmless particles as if they were deadly threats. The resulting allergic reaction leads to the classic signs of asthma. However, although much data supports the hygiene hypothesis for allergies, the same cannot be said for asthma. Contrary to expectations, asthma rates have increased drastically in urban areas in the US that are not particularly clean.