For example, a Poisoned Cascoon evolving into Dustox, who now has the Poison type. Since Poison types are immune to the status problem itself, what would happen?
>... Also, poisoned Pokémon who evolve into a Poison- or Steel-type Pokémon, such as Cascoon evolving into Dustox, will keep the poisoned status after evolving.
Source: Bulbapedia/ Poison