Zithromax is some potent (and expensive) stuff. It's the same drug they were prescribing for anthrax. You should allow it 2 weeks to work though, of course this is also assuming the the original source of the infection, like an abcessed tooth, is removed. It takes 3-4 days for most oral antibiotics to reach a full therapeutic level in the blood stream.

I have to chime in with Mark - don't extrapolate your cancer with her. It is not hereditary or contagious and many children have similar problems with their baby teeth. In some infections the lymph nodes under the armpits will swell up as well. Take a deep breath.