Would a polymorphed undead retain any diseases when it reverts back to normal form

Specifically, if I cast a heightened polymorph a lich, turning it into a sheep or caterpillar or something then cast contagion on it (5th level cleric spell that gives the target a disease of the casters choosing after it fails 3 CON saves) then revert it back to lich via damage, would it still carry the disease?