How can I treat an infected insect bite?

Depends on Severity. If the degree of inflammation is mild, topical polysporin (bacitracin and polymyxin) three times per day may resolve the infection. However, for a bite that has pain, swelling, redness, warmth, and/or a yellowish exudate, consultation with a physician for appropriate treatment including systemic antibiotics is indicated.
See below. See your doctor for an evaluation to see if the bite is truly infected and if so whether oral or topical antibiotics are needed.