User blog comment:Aramat24/Problems with mass effect/@comment-1388547-20120602195151/@comment-1679409-20120827162401

I second both of those answers.

1. Not only could the technology/care have improved to the point where it could improve Joker's condition, but he's now working for Cerberus, which obviously has the ability to bring a person back from the dead. So, it's not too big of a stretch to consider that maybe he could get better treatment with Cerberus than with the Alliance or as a human civilian.

2. That's the obvious explanation from a technical standpoint. I can't think of a good in-game explanation, though.