At the start of the game, you put one of them into play and shuffle the rest into the deck. Each disease has a different ability, and they affect all players. Disease cards don’t count as permanents, and whenever you draw one it goes into play immediately.
As your action in a turn, you can shuffle all Disease cards from play back into the deck – this is the only way to get rid of them, but it might be worth it…especially if all eight are in play!