Researchers are looking for aspects of lifestyle that can interact
with genes to cause or contribute to ALS. The notion that ALS could
be linked to viral infection is a concept that reappears periodically
when researchers focus on the issue. Exposure to toxins and the
inﬂuence of intense exertion are ideas researchers consider as
possible reasons for the ﬁnding that some veterans and some
athletes have increased incidence of ALS.