Everyone is right that you should not freak out until you get all the info from the doctors.
But lets assume you are going to be sick for a while. Once you get into that phase, figure out 2 things:
1) When can you be focused and productive? Work at those times.
2) How can you take care of yourself, stay sane, have fun, and not let a disease ruin your emotional stability. This is just as important,if not more so, than the work.
One last thing - this is your life we are talking about. Startups are just jobs. Keep your priorities straight as you make decisions.