The long version: If you went past a Still-Healthy Island Card, after following the Blight instructions, put it – original Healthy side up – atop the stack of Blight Cards (so you encounter it first, just like you did this game).
If you reached (but did not pass) a Still-Healthy Island Card, treat it as a Blighted Island Card for the Blight instructions – but in your next game, if you get past it to a new Blight Card, you will need to add new Blight from the box rather than from the pool by the side of the card.