Like other warts, flat warts are caused by the human papillomavirus (HPV), and are slightly raised, flat-topped and smooth growths that are a bit darker than your normal skin color. According to Warts.org, there are over 100 HPV varieties; HPV strains 3, 10, 28 and 49 have been found to be responsible for flat warts in particular. HPVs can infect the body in a number of ways and cause flat warts.
Poor Hygiene
One of the primary risk factors of flat warts appears to be young age, according to MedlinePlus, an online medical reference from the National Institutes of Health. Children and adolescents are the primary sufferers from flat warts, as they rarely appear in adults. This may be due to the fact that children often do not have good hygiene habits, and flat warts are highly contagious and can be easily transmitted from one child to another through either direct contact or by way of touching something that's been handled by an infected child.
Weak Immune System
A weakened immune system--whether due to immuno-suppressant therapies for such conditions as HIV/AIDS, drugs for organ transplantation, or very young or old age--may leave you susceptible for an HPV infection that will lead to the development of flat warts, according to the Mayo Clinic.
Cuts and Scrapes
Flat wart viruses enter the body through cuts and scrapes, so even shaving with an HPV-infected blade can cause flat warts if you nick yourself in the process of shaving. This may also explain the predominance of flat wart infections in children as opposed to adults, who usually do not get as many cuts and scrapes as children do.
