I don’t know about the legal side of not declaring such an illness, but I do know that if I ever felt the need to work again, there’s no way I’d not be totally honest with a potential new employer, especially if it were a small business.
I’d be thinking of all the probable days off ahead for treatment and how that would affect my colleagues, as well as a certain amount of days when I’d not feel well enough to do a full days work.
I’d feel terrible if I had to constantly let people down knowing that I knew it was likely to happen.
You don’t say what type of job it is, but I do think being open and honest is the best way to be and it then gives an employer a chance to decide whether they can afford to employ you.