Warts are caused by the human papillomavirus (HPV), which triggers rapid cell growth in the top layer of the skin. There are over 100 types of HPV, and different strains of the virus are responsible for different types of warts. HPV is easily transmitted from one carrier to the next but not everybody that comes into contact with it develops warts.
Children with underdeveloped immune systems or those with a weakened immune system are more prone to developing warts than healthy adults. It is important to treat warts as soon as they appear to avoid spreading them to other people.