My Girlfriend's Husband Has A Blood Cancer Disease. She Couldn't

My girlfriend's husband has a blood cancer disease. She couldn't explain it to me but wanted me to look it up but all I know it starts with an "M". I want to know more so I can be supportive to her. Her husband doesn't want her to tell anyone but she knows my brother past away from cancer so she is looking for some form of support. Can you tell me anything about "blood cancer" other than leukemia?