Cavity in the skin filled with fluid. Blisters are caused whenever one of the deeper layers of the skin is damaged, so that fluid can accumulate underneath it. There are various ways in which such damage may be caused, the commonest being pressure or friction from badly fitting shoes. Burn blisters are caused by second-degree burns. There are also various skin diseases linked with blistering, including pemphigus. Some insect bites may cause blistering but this is rare. The base of a blister is a piece of defective skin very sensitive to infection, which can easily set in if the blister bursts or is pricked. A closed blister disappears of its own accord, because the moisture is absorbed into the bloodstream. But if the size or position of a blister makes it very irksome it can be better to burst it, ideally as close to the edge as possible. After the blister has been opened with a sterilized needle the fluid can be carefully expressed, after which a sterile dressing should be applied.