You may find that this health question is preliminary and that if they wish to invite you for interview, they will send you a further, more detailed health questionnaire. I can understand your concerns but I wouldn't be too worried, after all, the incidence of diseases like asthma and diabetes is very high, and practically everybody knows that they are allergic to something, even if it's just pollen (hayfever!) so you won't be the only one. Good luck.