This is one of my biggest fears, getting fired. No they can't fire you because you have an illness, but if they really didn't want you working for them, there are other ways they can get rid of you. If you are really worried have a little word with your boss and explain how you feel. I'm sure he will understand your situation and worries. At least then you will know, and finally be able to get your self on the mend without this worrying you.