The bat people (1974)

Dr. John Beck, recently married, decides to take his wife, Cathy, spelunking in Carlsbad Cavern. While there, Dr. Beck, who specializes in bats, is bitten by a fruit bat. He is then, inexplicably, transformed into a vampire bat. While he escapes and seeks help from another doctor, it is clear the treatments are not working. In fact, they are aggravating his condition. Dr. Beck unwittingly goes on a killing spree, catching the attention of Sergeant Ward.