LOL I work for the insurance company, so let me clarify this.
It depends on what risks your policy covers. Luggage, liability, assistance services, accident? Not necessary to admit it.
Medical costs? That's where it gets tricky. You see, if you had a claim related to high blood pressure, heart attack or problems, eye problems, etc. and you didn't admit diabetes beforehand, they could call it an insurance fraud, since these things are related to diabetes complications.
Better check than be sorry later. They should have the insurance conditions online. Maybe it already is in their exclusions. And anyway, if it is important for them, there will be a question about it during the process of concluding your policy. If there is, don't lie. If there isn't - well, no one can blame you for not telling them voluntarily.