Check out the information that was sent to you. There should be something there regarding a waiver. My doctor filled out a waiver request (or wrote a letter -- it was so long ago I'm not sure) and I haven't received another jury notice since.
I sent in a copy of my disabled badge (the parking space one) and an old copy of a consultants letter. Then wrote a cover letter. They were fine. I'd be happy to do jury service but could not guarantee that I'd be well enough to do it. I cannot sit upright all day, cannot stay awake and cannot concentrate / understand all that is going on (even if I was well enough to leave my home).