Why do doctors worry about white blood count if you have an infection?

To monitor response! Wbc count, preferably w/ differential is an excellent way to monitor treatment response to antibiotics, or other treatments!
Treatment. Your white blood count shows how your immune system responds and what is the effect of treatment.

Related Questions

Low white blood count. Doc said not to worry but I am? Had sinus infection since June, herpes1 breakout that's gone, ear infections still have, now tremor

Check immunity. Hard question to answer without seeing and examing you. This could be nothing, but If infection persists or out of ordinary, check your immune system. Tremor can occur in some immune deficiency conditions or induced by antibiotics.

Doctor says white blood count slightly low should I worry?

Leukopenia. A low white cell count (leukopenia) can be caused by many different things. Some people have low white cells due to a medication they are taking. Others are born with it. Others develop it because of dangerous conditions such as leukemia. Please consult with a hematologist to determine what form of leukopenia you have.