What caused the death of Norman Warne, Beatrix Potter's fiancee?

There are conflicting opinions regarding what caused the death of Warne, fiancee to Beatrix Potter (who wrote "The Tale of Peter Rabbit" and is the subject of the recent movie, "Miss Potter"). Some sources declare him to have died from leukemia, wheareas others state that pernicious anemia killed him. What I know about his illness is that it came about suddendly and, in the term of four weeks, ended his life. Can anyone clear this up? Please offer proof of your theories and statements.