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It can be understood from the passage that allergies ----.
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.