Very painful sterile inflammation of the skin caused by heat or chemicals. There are three degrees of burn. In a first-degree burn the skin looks red, is very painful and may be slightly swollen. Only the uppermost part of the skin is affected. In a second-degree burn, the skin is also blistered. Infection can occur, and scars may form when the burn heals. A third-degree burn extends into connective or even muscular tissue. This is no longer painful, because the pain nerves are also burned. A third-degree burn is generally surrounded by a border of second-degree burn, in its turn surrounded by an area of first-degree burn. With every burn the area of burnt skin is significant; if this area is too large, much fluid is lost through the wound and shock can occur. A first- or second-degree burn usually recovers without a scar forming. However, third-degree burns usually leave disfiguring scars.