Difficulty level - very hard + hardcoe
This never happened prior to the latest patch. Basically, once your companion gets stung by a cazador, he's a dead man as his health keeps diminishing even after a battle is over. Also, if you use a stimpack on him he dies instantly

Anyone else experience this?

Got the same thing happening here. Boon is losing health long after battle is over and finally dies. Tried to tell him to use a stimpack through the companion wheel, but then he just dies instantly...

LOL Anaphylactic Shock!

Indeed. It's a game breaking bug for me as I haven't made a proper save in ages, and both my auto and quick save are after my companion got poisoned...

If you're not playing hardcoe it's not that big a deal. Annoying and irritating but eventually they recover. On hardcoe, tho...yes, it's very bad. heh. Only thing to do is tell them to Wait and kill the critters yourself.

http://www.gamesas.com/index.php?/topic/1156069-boon-dies-for-no-reason/page__view__findpost__p__16927689 explains the bug, and it's happening to a lot of people so yeah...quite common.

Just to say, if you manage to use a stimpack before their health is too low, they'll survive the HP loss from the bug. But that's when they're higher level/have more hp's plus, of course, dependent on you reaching them in time.