Gilligan is bitten by a bat, and then dreams that he has turned into a vampire. Fortunately the Professor knows about bats (what DOESN'T he know about?!), and is able to identify the bat that bit Gilligan as a common red fruit bat, rather than a vampire bat.
Keywords: boat-travel , campy , cheerful , clumsy , comforting , deserted-island , hilarious , island , near-hawaii , shipwreck , sitcom , uncharted