Jack was severely malnourished and he was also dehydrated (that means he hadn't had any water for a long time!)
Read about how Jack was found @ NY Times.
What happened next was he was emergency rushed to BluePearl hospital in Manhattan (a really high-class place). He was treated by a team of cat specialists. They put a feeding tube in him, juiced him with intravenous fluids, antibiotics and Jack got an operation if some kind - details are sketchy on this.
|So far ... so good. Or so you'ld think|
Jack the Cat had gone for so long with no food or water that his organs were dysfunction and his skin was so thin that blood couldn't flow into it. Despite all these efforts and love from his fans, 12 days after Jack crashed into Customs, Jack had to be put to sleep. American Airlines could have done more. More details.