Infections are serious, and can get out of control quickly, especially with a weakened immune system, so it's best to go to the doctor, have blood work, and get proper treatment wether it be bacterial, viral, fungal infection or combination. As far as time, it may take a while for your immune system to bounce back, and should be monitored with blood work, usually by your oncologist. There are immune boosting foods, vitamins, having less stress, adequate sleep may help, even "light" exercise, yoga, meditation. Too much exercise drains the immune system.

I hope you, and your family feel better.