Medical malpractice occurs when a person is injured by the neglect or substandard service provided by a physician or other health care professional, hospital or institution. If you are injured as a result of a preventable error or because of negligent care when you receive medical treatment, you may have a medical mal-practice case.
However, just because you are injured during medical treatment, you do not necessarily have a medical malpractice case that can be won. For example, there are some cases where a person suffers an injury from a complication during the medical treatment, but that complication is considered "normal". An example of this would be infection after a surgery that is considered common risk. If the complication is considered "normal", the injury was not the result of negligence and there is no medical malpractice case that can be won.