I feel obliged to provide an answer to this question, as I have literally gone through the same thing. In the beginning of my 4th year as an applied math major, I was diagnosed with leukemia. While I initially hoped that I could avoid changing my class schedule and deal with cancer "on the side," it soon became clear that this was not going to happen. I dropped every class I was taking except for senior design (which the school thankfully worked very closely with me on), failed my MATH GRE subject test, and canceled both my application to the NSF Graduate Research Fellowship Program and applications to math graduate schools in general. After 8 months of hardcore chemo treatments and 2 years of monthly chemo, I finished my treatments, and was ready to apply for graduate schools again. [I had a nearly full-time job for those 2 years in a somewhat related field]
When I was applying to grad schools, I confronted the cancer topic in the opening paragraph of my personal statement. I didn't present it as a reason for someone to take pity on me though. Rather, I presented it as a personal challenge that derailed my initial plans, but ultimately made me a better researcher (nay, person!) after I overcame it. I briefly mentioned how the experience forced me to alter the courses I was enrolled in, explaining the WITHDRAWS on my transcript, but focused more on how during these treatments, I still came to campus as an academic tutor for fellow undergrads.
I am afraid that if I tell the admission committee that I suffered from cancer, they will reject me because they may think that if it comes back during my graduate studies, then the same thing could happen. Will it look really bad that I only completed very few subjects in the first four years?
The professors who read your application are not heartless. They'll understand that your first few years aren't representative of your potential. Your 4.0 and consistent academic progress after you defeated cancer are proof enough that you are serious about your studies. Focus on this evidence and talk about your passion for your math interests, and you should be fine. Getting cancer doesn't ruin your life forever, I promise.
Thankfully, I have been in remission for the past 5 years now. But even so, I am confident that both my advisor and grad school would give me the time off to handle health issues if it comes back. I'm sure you'll have the same experience as me, in that regard. Good to hear you're back on your feet, and good luck with your applications.