Last Monday, Roman Reigns announced that his leukaemia had gone into remission and in a special episode of WWE Chronicle, The Big Dog has detailed what his recovery entailed.
Apparently, he found out that something was wrong when WWE doctors informed him that his blood tests were coming back abnormal and a further test confirmed that his white blood cell count was elevated. Reigns added that he wasn't able to move much during the first few months of treatment and he didn't start training until he "got his feed under" himself again.
As for the type of cancer he had, it was CML Leukemia. As it was caught in the early stages, he was able to take a pill and undergo oral chemotherapy. While that had side effects, it was still a better option that radiation (and explains why he didn't lose his hair).
When Reigns relinquished the title, he knew that it was possible he would never return to WWE but once he was cleared, he contacted Vince McMahon and started working towards a comeback.