Why was Kelly unable to devote much time to her children lately and why was she afraid of catching infectious diseases?

Kelly felt gratified when she was able to comfort Claire through a rough night of teething. She hadn't been able to devote much attention to them since she was usually sleeping or drowsy. Kelly was experiencing severe bone pain and nausea in the mornings. There was an outbreak of West Nile virus and with her weakened resistance she feared she would contract it. Edward reminded her that she would be done with chemo by Christmas.