Can a company fire someone due to lack of performance caused by a kidney disease?

I was diagnosed with a kidney disease about two years ago. I have been stable for the past year and a half. I started a new job November 5, 2007 as the Client Services Manager for a local company. In January, my kidney function dropped from 40% down to 25%, and blood pressure skyrocketed. The lowered function causes exhaustion, and problems with concentration/memory which in turn caused performance problems at work. I had been going to my doctor who was working on stabilizing my condition. Yesterday I was let go due to performance issues. Apparently my director didn't inform the HR department that I had a disease causing the problems, and when I brought it up, her eyes got big and she looked over at my director and said she was never told about this. Before she could say anything else, my director said she would walk me over to my desk so I could get my things, and then she would walk me out of the building. Are they allowed to do this?