Hi. my son has these wart like pimples growing under his left arm. it is spreading and i am getting concerned?

It happens: The ones that spread easily are a cousin of the wart called molluscum contagiosum. They are usually smaller, and may have a dimple in their top. These can fall off over time (6m-1yr) without treatment. They could also be warts that last longer. Treatments always leave small scars. You should confirm the process with the doc. They are not a threat to the kid.