#1 Don't wait if you suspect an infection from a cat scratch or bite. It can proceed into a systemic infection called cellulitis in a matter of hours. Once it gets to that stage, the patient is usually pretty sick. Clean it and get a Neosporin type antibiotic on it ASAP. Keep it clean and open. If you should begin to run a fever or notice any red streaks on your leg other than the scratch get to an ER or your Doctor STAT. Time is of the essence.
#2 Hydrogen Peroxide is a great antiseptic, but it doesn't foam because there is infection present. The reason why it foams is because blood and cells contain an enzyme called catalase. Since a cut or scrape contains both blood and damaged cells, there is lots of catalase floating around.
When the catalase comes in contact with hydrogen peroxide, it turns the hydrogen peroxide into water and oxygen gas. The release of the oxygen gas creates the foam.
Please don't wait on this. I don't want to alarm you, but Jeff was bitten by one of our elderly cats a few years ago and came within just a few hours of losing his hand and part of his arm. We see about 2-3 cases of cellulitis a week through the ER, and they're hard to treat effectively without the use of heavy duty antibiotics like clindamycin, and failing those being effective, surgery becomes the only option.