The Lost Island - S5.E16

While searching for a fallen satellite in the Pacific, Steve rescues a drowning victim who turns out to be an alien. She offers to help him acquire the satellite, which has landed on her race's invisible island. However, many of the island's populace in fear of a revolution, yet are unable to leave due to a lack of an immune system to Earth illnesses. Steve must get an experimental serum for her before she will help him.