How would you impress upon someone the importance of avoiding bacterial infections to a person with leukemia?

Immune system down. Individuals with leukemia, due to their cancer and the treatment for it, have very impaired immune systems that make it very difficult to fight off infections - bacterial, viral, fungal. That is why people are being treated for this cancer and many others often are on antibiotics, antifungals, antivirals to help prevent these devastating complications.