The timestream was changed by sending a Doomsday Machine back in time. There,...
Star Trek Phase II
Season 1 Episode 5: Blood and Fire: Part Two
Spock, Peter Kirk & Alex Freeman are stuck aboard the U.S.S. Copernicus which is invested with Regulan Bloodworm and has to be destroyed. However, they meet two researchers aboard who claim to have a cure for the infestation.