Pretty easily — impetigo is very contagious. Your baby may have picked up the bacteria by touching an infected child or some object that the child touched, like a toy or a towel. Or he may have already had the bacteria on the surface of his skin and then gotten a cut, allowing the bacteria to enter and cause an infection.
In addition to cuts and scrapes, the bacteria can invade the skin through cold sores, eczema, an insect bite, or other areas where your child's skin is damaged or sensitive. He may get it right below his nose if it's sensitive there from wiping away mucus. Impetigo is more common during warm, humid weather.