What Causes The Lowering Of The WBC Levels?

My mom is in icu. She is intebated, She is 85 and weighs approx 205. She has COPD Shes been in the hospital for 4 weeks this visit. The reason we took her to the hospital is she had a fever. Her white blood cells were very low and the doctors told us her immune system is comprimised. So she went to reverse isolation in the hospital where the dr's tried to locate the infection and gave her iv antibiotics ,and meds to raise her white blood cell count. The infection that is somewhere in her body as yet the doctors can't locate it, She is still and has been on IV anitbiotics for many weeks. Her fever went down and her white blood cell count improved. So she was moved to another part of the hospital. Then after a few days she began having trouble breathing and her heart rate rose dangerously high so they intebated her. They also told us she was Septic/sepsis. So again to ICU. Today will be the 12th day and they are going todo a trach today. Also her kidney functions arent good. Now I'm to learn her arms are weeping. So I'm thinking CHF. My sister-inlaw believes she'll pull out of it. I'm not so sure. any words of wisdom would be appreciated.