A burn is an injury to the skin or flesh caused by heat, electricity, chemicals, friction or radiation. There are three degrees of burns: first-degree burns damage the outer layer of skin, second-degree burns damage the outer and inner layers, and third-degree burns damage the deepest skin layers and tissues below. Burns can cause swelling, blistering, scarring, shock and even death if severe, and increase risk of infection by damaging the skin's protective barrier. Treatment depends on the degree of the burn, with first and second usually healing without grafts but third often requiring skin grafts.