You are an intern at the Denton Department of Health. Your supervisor, Dr. Rodriguez, has informed you that there is an outbreak of severe acute respiratory syndrome (SARS). 

She asks you to go to Denton General Hospital and also survey another hospital in the Denton suburbs, Rothman Hospital, to study cases there.  At these two hospitals, you discover 178 cases were treated, some of whom were residents of a nearby apartment complex, Happyland apartments, and others of whom appear to have caught SARS at the hospital. The cases at Rothman hospital were housed in several wards: the infectious disease ward, the medical ward, and the surgical ward.  Rothman hospital in particular seemed to have an explosion of cases in their medical ward. Your first step is to take measures to control the outbreak, via isolation of cases and the use of personal protective equipment (PPE) by hospital staff.

What is the definition of a case-control study?