Multiple sclerosis primarily affects adults, with an age of onset typically
between 20 and 50 years, and is more common in women than in men. The cause
of this disorder is not known, but environmental, viral, and genetic factors
are thought to play a role. The January 26, 2005, issue of JAMA includes an article reporting that exposure during early childhood
to infant brothers and sisters is associated with a reduced risk for MS in
later life, possibly due to alterations in immune functions.