All warts -- including flat warts -- are caused by a virus known as human papillomavirus, or HPV. There are dozens of kinds of HPV, and the different warts caused by it tend to appear on various surfaces of the body. For the most part, flat warts are caused by HPV types 3, 10, 28 and 49. HPV is commonly transmitted by direct contact from one person to another. It's also possible for HPV to be transmitted indirectly. So, if you come into contact with the virus in a locker room, pool or other public place, you may get warts. In addition, once you have a wart, it's very easy to spread the virus from one place to another on your own body [sources: Intelihealth, Warts Information Center].
You're more susceptible to HPV if you've got a cut, scratch or some other opening in the skin. If you have a scratch or cut and it gets infected with HPV, it's likely that flat warts might appear along it. If you already have flat warts, be aware that they can easily spread to other areas of your body that may be scratched or cut [source: Merck].
Another way that flat warts spread is through the act of shaving. Shaving with a razor is a daily routine for many people. However, since there's a chance you may nick yourself with the razor, shaving may encourage the spread of flat warts. For men, that means an outbreak on faces, and women may get them on their legs [source: Warts Information Center].
If you have a strong immune system, you already have a line of defense to combat flat warts. However, because warts are caused by viral infection, people whose immune systems are compromised by an autoimmune disorder or who are taking certain medications that weaken the immune system are more likely to get them.