Flat warts are small, painless growths on the skin. They most commonly develop on the face but may also manifest on the legs. This type of wart is much smaller than common warts and often remains smooth, almost flush to the skin. When you develop a flat wart, it's rare for only one to form. The American Academy of Dermatology explains that flat warts typically grow in numbers--anywhere from 20 and 100 can affect a section of the skin at a time. Like most warts, a number of contributing factors play a role in their development.
Human Papillomavirus
Often referred to as simply HPV, the human papillomavirus causes flat warts. But not just any strain of HPV produces this type of wart. According to the Merck Manuals, an online medical library for health-care professionals, HPV 3, 10, 28 and 49 are the strains responsible. If you come into person-to-person contact with someone infected by these strains, you can develop flat warts roughly two to six months after exposure.
Immune System
Just being exposed to HPV doesn't necessarily mean you'll develop flat warts. Your immune system also plays a significant role in their development, according to the Mayo Clinic. Some people's immune systems respond to the virus in a way that doesn't allow the warts to form. For others, the body's immune response may not be strong enough to fight the infection, which leads to the small, painless growths forming on the skin.
Autoinoculation
Autoinoculation is the process of removing micro-organisms from one part of the body and introducing them to another. According to the Merck Manuals, this is the way you cause flat warts to spread. Scratching areas of the skin can cause flat warts to expand from one area to the next. Shaving can also introduce HPV to other sections of the skin. Both scratching and shaving can create tiny breaks in the skin large enough to reintroduce the virus into other areas of the body.
