"She has cancer" is a specific event, so we logically deduce that it is "a type of cancer". When you say "AI will cure cancer" that is not a singular event, so

"She has cancer" is a specific event, so we logically deduce that it is "a type of cancer". When you say "AI will cure cancer" that is not a singular event, so you assume cancer in the plural. You would have to say "AI will cure some forms of cancer" if you didn't mean the plural here.