Common warts are caused by the human papillomavirus, or HPV, which infects the top layer of skin and triggers an overgrowth of tough, hardened skin cells. About 10% of the global population has warts at any given time, and among school-aged children, that figure rises to 10% to 20%.
The Virus Behind Common Warts
HPV is a large family of viruses with over 200 known types. Only a handful of those types cause the rough, dome-shaped bumps most people picture when they think of warts. Common warts (the kind that typically show up on fingers, hands, and knees) are most often linked to HPV types 2 and 4, though types 1, 27, and 57 can also be responsible. Different HPV types tend to prefer different body sites, which is why plantar warts on the soles of the feet, flat warts on the face, and genital warts are each associated with their own subset of strains.
How HPV Gets Into Your Skin
HPV cannot penetrate intact, healthy skin on its own. It needs a way in. Tiny cuts, scrapes, hangnails, or even the microscopic cracks that come from dry skin give the virus access to the deeper cells of the outer skin layer. This is why warts so often appear on hands and fingers, areas that are constantly exposed to minor injuries.
Once inside, the virus hijacks the skin cells responsible for producing keratin, the tough protein that forms your skin’s protective outer surface. Infected cells begin dividing faster than normal and piling up extra keratin, which is what creates the hard, raised bump you see on the surface. The tiny dark dots sometimes visible inside a wart (often called “seeds”) are actually small blood vessels that have clotted, a result of the rapid tissue growth feeding the wart’s blood supply.
Warts don’t appear immediately after exposure. The incubation period varies widely, but it generally takes at least one to several months from the time HPV enters the skin until a visible wart forms. In some cases, the virus can sit dormant much longer before producing a noticeable growth.
How Warts Spread
HPV is contagious, and warts can spread through both direct and indirect contact. Direct spread happens when you touch someone else’s wart or make skin-to-skin contact with an area carrying the virus. Indirect spread happens through shared objects: towels, razors, nail clippers, or even gym equipment that has come into contact with a wart.
You can also spread warts to other parts of your own body. Biting your nails, picking at a wart, or shaving over one can transfer the virus to new sites. Moist or damaged skin picks up the virus more easily, which is why communal showers and pool decks are common sources of plantar warts on the feet.
Why Some People Get Warts More Easily
Most people encounter HPV at some point, yet not everyone develops visible warts. The difference comes down largely to how your immune system handles the virus. A healthy immune response can suppress HPV before it ever produces a growth, or clear an existing wart over time. About 40% of warts in children disappear on their own within two years without any treatment, purely because the immune system eventually recognizes and eliminates the infected cells.
Several factors tilt the odds toward developing warts:
- Age: Children and teenagers get warts far more often than adults, partly because their immune systems haven’t yet built up defenses against the many HPV types circulating in schools and playgrounds.
- Skin barrier damage: People with eczema (atopic dermatitis) have a higher risk of cutaneous HPV infections because their skin barrier is chronically disrupted, giving the virus easier entry.
- Weakened immune system: Conditions that suppress immune function dramatically increase wart risk. This includes HIV (which depletes the specific immune cells that fight viral infections), autoimmune diseases like rheumatoid arthritis and lupus, and medications used after organ transplants or during cancer treatment.
- Frequent skin trauma: Occupations or hobbies that involve repeated cuts, abrasions, or wet skin (meat handling, swimming, manual labor) create more opportunities for the virus to take hold.
There are also rare inherited immune conditions that make people exceptionally vulnerable to HPV. One example is a condition called WHIM syndrome (the “W” literally stands for “warts”). Another is a group of genetic mutations affecting immune cell signaling, where up to 50% of affected individuals develop persistent, hard-to-treat warts. These conditions are uncommon, but they help explain why some people struggle with warts that keep coming back despite treatment.
Why Warts Come Back
One of the most frustrating things about warts is their tendency to recur. This happens because most treatments, whether over-the-counter salicylic acid or in-office freezing, destroy the visible wart tissue but don’t necessarily eliminate every virus particle in the surrounding skin. HPV can persist in cells that look completely normal, and if your immune system doesn’t fully clear the virus, a new wart can grow in the same spot or nearby.
People taking immune-suppressing medications face the greatest challenge here. Organ transplant recipients, for instance, often develop widespread, stubborn warts because the drugs keeping their body from rejecting the new organ also prevent the immune system from controlling HPV. In these cases, warts tend to multiply over time rather than resolve on their own.
For most healthy people, though, the cycle does eventually end. As your immune system builds a stronger response to the specific HPV type causing the wart, recurrences become less likely. This is also why adults tend to get fewer warts than children: years of exposure have trained their immune systems to keep the virus in check.
What a Common Wart Looks Like
Common warts are firm, raised bumps with a rough, grainy surface. They range from the size of a pinhead to about the width of a pencil eraser, though they can grow larger or cluster together. The surface often looks grayish-brown or skin-colored, and if you look closely, you may notice the tiny dark dots (thrombosed capillaries) that distinguish warts from calluses or other skin growths. Unlike calluses, warts interrupt the normal lines and ridges of the skin. If you look at the fingerprint pattern on your hand, it will detour around a wart rather than passing through it.
Warts are usually painless on the hands and fingers but can be tender when they develop on weight-bearing areas like the soles of the feet. They feel hard to the touch because of the extra keratin buildup, and their surface can be slightly cauliflower-like when they grow larger.