A good portion of the patients who come to me complaining of warts don't actually have them. A very common type of callus looks very similar to a wart, and often times it takes a podiatrist to tell you which one you have after trimming away some of the overlying tissue.
Warts are caused by a virus that lives in the most superficial layers of your skin: the layers without blood vessels that give us our "water-proof" covering. The virus effectively hides there because our immune system uses blood vessels to get around, and if there is no blood flow to a particular area it's very difficult for our infection-fighting cells to even know something is wrong.
Occasionally warts can spread, and often times they will get bigger over time. They are typically seen in younger patients (under 45 years old), and can be very painful when squeezed, but are not as tender with direct pressure. They can be destroyed with topical treatments, but it may take several visits. Very rarely they require surgery for removal, but only in very severe cases.
On the other hand, you may have a nucleated callus with a core in it--they look very similar to warts. The technical term for these is long and unwieldy--intractable porokeratosis--so I generally shorten it to "IPKs". This type of callus is caused not by shoes or external sources, but from internal pressure. They usually show up under a knuckle in the ball of your foot, but can occur anywhere on the sole. Another factor in the formation of IPKs is that sweat glands in your feet can get plugged, and once that core forms, Mother Nature takes over and surrounds the painful area with layer upon layers of callus. Usually Mother Nature is smart, but in this case, she's working against us--the layers of callus tissue just make that plugged sweat gland more painful.
IPKs are usually tender with direct pressure rather than squeezing, and can be diminished with topical treatments, but when internal pressure from bone spurs or weight bearing knuckles are the root cause, topical treatments and trimming alone won't make them go away forever. Unfortunately, with calluses, unless you modify the internal pressure (we're talking surgery at this point) they will likely continue to come back. Your podiatrist will have maintenance options for you, though, so if you're not wild about operating rooms there is still hope for those feet!
Bottom line: before you start using over-the-counter topical irritants like corn or wart removers, see your podiatrist to determine whether your problem spot is a wart or a callus. We'll work with you to find the right treatment for YOUR particular problem!