In late 1949, things were looking bleak for Chiang Kai-shek and his Nationalist forces. Two million Nationalists had just fled from the Chinese mainland to Taiwan, and now the Communists looked poised to invade the island. Despite the fact that Communist forces had experience with amphibious operations and that a Communist invasion of Taiwan would likely have been successful, the operation never happened. What was the reason for Taiwan’s salvation? According to one source [gated], it was a waterborne parasite: schistosoma japonicum.
How Swim Lessons And A Disease Saved Taiwan | U.S. Naval Institute