how do you stop a plague?

[Dracus]

You have to build a ship, or you can let all the pop die and then repopulate the planet. It seems that in SEIII the plague stayed with the planet until cured so all new population would still die, but in SEIV it seems that the plague dies after all the population dies.
Had this happen as a radom event and by the time I got my ship built, the planet was dead. So I dropped some more people and they were fine. DID not need to use the med ship at all. So I never build them any more. I just let the people die and pull more from a over populated planet.