The medical history form is designed to determine if you have any medical conditions which might pose added risks to you as a recreational diver. You should answer the questions honestly (not "supposed" to answer as indicated by another poster) -- they are for your safety and not the instructor's.
If you answer "yes" to any question on the form, then you will need to get a medical release signed by your doctor indicating that he has reviewed the issue and feels you are clear to dive. Or, perhaps, the two of you have worked out parameters that would permit you to dive more conservatively.
In any case, if you require a medical release to get certified, then I would HIGHLY recommend you carry a copy of it with you anytime you go diving. Keep a signed original in your divelog and get a new release signed each year when you get your physical done. Then, when you get to your dive destination, you should be sure staff at the destination are aware of any risk factors you have and let them make a copy of your meidcal release... this ensures that they are aware of your risks so they can help you should you need it and, at the same time, ensures that you have the paperwork you need to be able to dive.
I have never seen a diver who had a signed release less than one year old rejected from any dive. And, of course, the release is only needed IF you answered yes to any of the medical history questions.
