Must I tell my prospective spouse about my illness or disability?

Yes, a defect or illness that affects the marriage must be disclosed; concealing it is deception.
The jurists list several 'aib (defects) that allow the other party to seek judicial dissolution (fasakh), such as illnesses that prevent marital relations and serious contagious diseases. More broadly, concealing major matters such as a chronic illness, known fertility problems, a serious mental disorder or an addiction is contrary to trustworthiness. Disclosure need not be public; it is enough to tell the prospective spouse and their wali privately. Pre-marriage health screening in Malaysia helps, and its results should be shared with each other. See judicial dissolution (fasakh) and khitbah.