Cancer in and of itself in uncountable. However, this is not the case when referring to different types of cancer. When referring to different types of an uncountable noun, the definite article is used. Observe:
Leukemia is a cancer.
Lymphoma is a cancer.
In the image above, the word ‘cancer’ is pluralized so as to highlight the fact that there are different types of blood cancer. (Rule 3.1 Exception #1)