Crowded, unsanitary living areas predispose to body lice
, although anyone can catch them. Barracks, dormitories, refugee camps & the homeless are typical modern day ways to get infested.
Body lice have been around for a very long time and are good survivors. Treatment is effective if you are thorough with prevention. See mayo clinic website http://www.Mayoclinic.Com/health/body-lice/ds01073.