If you had cancer, you'd go through sick leave, vacation, short term disability and FMLA. To minimize leave days, you still have to work while you are not too sick to work. You might ask for telework, flex time, that's reasonable accomodation for disability.
If you work for public institution or big company, your risk of losing your job due to illness is less than when you work for a mid-sized or small business.
Of course, all these doesn't gurrantee that you will be employed when you die of cancer. So if you are worried, buy term life.