The Plague was never going to be an "official" expansion, at least not official by Hans im Glück standards. Kettlefish has even stated that they didn't especially like that expansion. Rio Grande Games separately negotiated a contract with Spielbox to release Plague and Tunnel (alongside Corn Circles I, which that already had rights for). But the expansion died after Rio Grande lost its rights to Z-Man Games.
Regarding the expansion, I like it quite a lot, but it can be vicious, especially in 2-3 player games. I agree with Tobias that it doesn't play well with others. In fact, in a game of mega Carc, the Plague would probably end up migrating all over the board, which could be fun to watch but would make the game especially difficult. Not all the FAQs for the expansion have really been resolved, either, since HiG won't comment on it. It doesn't get played much, but when we feel like an especially odd and aggressive game of Carc, we'll usually do Plague before we do Towers. And yes, I stick the two in the same category: both are fun, but both can be very challenging too.