A bite or sting can become infected within a few days, especially if your baby scratches it. If you notice swelling or a spreading area of redness around the bite, or if your baby has a fever, give his doctor a call. She may decide to put him on antibiotics. Be especially diligent about keeping your baby's nails short in the summer months, so he won't scratch away.
If a tick bites your baby, watch for signs of Lyme disease — in particular, a circular rash or a ring around the bite. Take your baby to the doctor immediately if you notice this telltale sign.