Dean
This is a common problem in Oncology. Many chemotherapy drugs work by destroying rapidly growing cells like cancer cells. However, other cells also grow fast like the blood cells in the bone marrow. When these cells are destroyed the white blood cells that fight infections are damaged. This can lead to infections that are very hard to cure.
Bone marrow transplants are another treatment that has a high mortality rate.
One day we may be better able to kill cancer cells and not harm normal cells.