Body dysmorphic disorder (BDD) is a mental illness where people think they look different to how they really look. People are usually said to have BDD if they are extremely critical of their body, even though there may be nothing noticeably wrong with it. If there is something wrong with their body, it is usually so small that a person without BDD would not be unhappy about it at all. People claim that they are insecure, but BDD is far worse.