Plantar warts are benign skin growths that appear when the human papillomavirus (VPH) infects the top layer of the skin.
They're usually small, although some warts grow bigger. Also sometimes they grow in clusters; these are called mosaic warts.
Sometimes corns and calluses are mistaken with plantar warts, because both of them appear in pressure areas and they usually get covered up with hyperkeratosis. We can distinguish them by evaluating if the pain is bigger when we pinch the zone or when we put pressure on it. Normally the pain is worse in warts when we pinch them.
This situation it's very common, especially in children, virtually everyone will have a wart or several of them somewhere at some time in their lives. They spread from person to person or indirect. Normally we can catch them when we share shoes or when we use public showers with people that are infected. That's why podiatrists always recommend that you do not share shoes and that you always use flip flops in public showers.