What leukemia is the weakest of them all?

my friend was diagnosed with leukemia about a month ago. She has Sisticphybrosis (i think thats how you spell it) as well so does that effect her chances of getting it to. She told me the drs are certain shes not going to die from it and that its the weakest of them all and shes not having chemotherapy but she didnt answer me when i asked her what sort it was so i left it because ididnt want to upset her. has any one got idea of what sort it is?? thanks.