The human papillomavirus (HPV) is the culprit in all wart contagions. The virus has more than one hundred known strains and infection by many of these can cause warts. The Centers for Disease Control estimates that 90 percent of people infected with HPV will develop immunity against the virus within two years.
Children (whose immune systems have not fully developed and who often have cuts and scrapes on their knees and legs) will most likely develop warts more often than adults.
Surface Contact
The virus thrives in warm, moist environments, making community swimming pools, locker rooms and bathrooms ideal places for it to live until a host picks it up. In order to infect a new individual, HPV needs entrance into the body. In most cases, the virus enters through a break or tear in the skin's surface.
The Merck Manual notes that the areas of the body commonly injured, including the legs, knees and feet, are most susceptible to infection from HPV. Children, who often run around barefoot on rough poolside surfaces and abrade their feet or who fall and skin a knee, can pick up the virus from an infected surface.
Towels and clothing, too, can harbor the virus. Children, who share an infected individual's towel or clothing, can pick up the virus if they have a cut or scratch on their own legs.
Autoinoculation
Warts are friable, meaning that small pieces easily break off and crumble and, once HPV has infected a child, autoinoculation or self-infection, commonly occurs. Picking, rubbing or scratching a wart on a knee, for instance, transfers the virus to the child's hand. Scratching the opposite knee, then, easily passes the virus to the new site.
Medical News Today explains that children have warts more often than adults because they have not yet become immune to HPV. That being said, flat warts are a specific variety of wart that children get more often than do adults. Tiny, flat, and somewhat darker than the surrounding skin, flat warts often develop in clusters of up to one hundred warts at one site. Flat warts will sometimes appear in a line, most likely the result of the infection sprouting along the edges of a scratch.
Person to Person Transmission
Medline Plus points out that person to person transmission of HPV is common. Children in close contact with one another, and who have scrapes and cuts on their knees and legs, typically spread the virus to each other. Touching or picking at a wart can spread the virus from child to child. Jungle gyms, playgrounds and other areas where children play can be fertile grounds for contagion. Unfortunately, because warts can take up to six months to develop, knowing who carries the virus or where one contracted it becomes difficult.
