Visceral larva migrans

Visceral larva migrans describes a number of conditions caused by infection with a parasitic worm that does not use human beings as a natural host. Once "trapped" within a human body, the worm will often work it's way out of the intestines, into the bloodstream, and will attack the organs, causing inflammation and damage that are atypical of a parasitic infection that tries to preserve the host as an intermediary. 
The majority of persons who develop the condition are under four years of age. Infection can be passed by ingesting a small number of parasitic eggs. 
There is disagreement about which non-human parasitic diseases can actually cause the condition. 
Visceral larva migrans at Wikipedia