Filiform wart on the knee of a 71 year old female patient. Warts are small lumps that often develop on the skin of the hands and feet but can occur anywhere in the body. They vary in appearance and may develop singly or in clusters. Warts are caused by certain strains of the human papillomavirus (HPV). There are lots of different strains of HPV and these cause different types of warts. Filiform warts are those with small, narrow, finger-like projections. Warts often clear up on their own without treatment however salicylic acid creams, cryotherapy and chemical treatments can be applied to warts to kill the affected skin cells.