can a seller cancel a contract if he is too ill to fulfill it

I'm sorry to hear.  You're certainly in a tough situation.  Without seeing the contract, it would be difficult to say your strength under it.  My inclination is that considering the illness a force majeure may be a tough stretch.  However, that is not to say you don't have defenses.  In your description, it sounds like you answered the summons on your own.  Litigation, particularly (in my experience at least) real estate litigation, can be very complex.  I would highly advise making an appointment with a litigation attorney to discuss all of the details of the situation and determine what options you have and the relative strength or weakness of these options.  Sounds like the case is already moving along so I wouldn't wait - even if you don't hire an attorney, you really need to get a legal opinion based on all of the facts (and the best place to discuss is in a confidential environment).