Murray Leinster's "Tallien Three" aka "The Hate Disease" (novelette, epidemic, free): Even mundane may be essential to life
When Calhoun, the Med Service man, reached the world Tallien III for a routine public health inspection, he found the world in a state of civil war. With society bitterly divided between "normals" & "paras".
Paras are afflicted by a highly contagious disease - an extreme urge to eat certain squirming animals! I couldn't quite see the logic of this "disease" & extreme horror associated with it that is threatening the world, but it's a story from an earlier era; & this is the only flaw.
Of course, the brave hero will get drawn in the local issues, will end up rubbing the powerful in the wrong places, become hunted, & generally have an adventure. And, of course, he will find a cure - a humble ingredient of the environment on earth since forever has been missing from the the local environment, & there is a simple way to introduce it...
Fact sheet.First published: Analog, August 1963.
Download full text in 4 parts from Webscription: 01, 02, 03, & 04.
Listed among the stories from John Campbell's Astounding/Analog.
Related: Stories from Analog/Astounding; Stories of Murray Leinster.