What is the hygiene hypothesis?

In the late 1980s, it was noticed that large families seemed to have
much less in the way of atopic disease. This was mainly found when
studying asthma, for which the link is much more established than
it is for atopic eczema. (The link does not apply to other types of
eczema.) It is thought that allergic diseases might be more likely to develop
if the immune system of an infant or young child is understimulated
from a lack of contact with infection, dirt, dust etc. The immune
system is designed to allow us to develop protection against
infections, so if a child is not exposed to any, the immune system
finds something else to react to! No single, specific infection has
been linked to atopic eczema, so this is not an argument against
immunisation, but it is worth avoiding unnecessary courses of
antibiotics in early life and trying to be relaxed about children
playing together and sharing germs and dirt. Perhaps the old saying
about ‘eating a peck of dirt’ has some validity. Some further support
for the hypothesis has come from developing countries, where
improvements in hygiene, and perhaps a better availability of
treatment for worms and other gut parasites, seem to be linked to
an increase in atopic disease.