Kaposi's Sarcoma (KS) is a type of cancer that forms in the lining of blood and lymph vessels. It typically appears as tumors on the skin or on mucosal surfaces such as inside the mouth, but it can also affect other organs like the lungs and gastrointestinal tract. The disease is caused by the Human Herpesvirus 8 (HHV-8), also known as Kaposi's Sarcoma-associated Herpesvirus (KSHV).