Medical specialists are experts in certain fields of medicine. They either treat specific parts of the body, such as the back or the brain, or they specialize in certain diseases, such as cancer. Family doctors keep a list of local specialists and can help patients choose the right specialist for each medical issue. In many cases specialists require a referral from a family doctor before they will see a patient.
Reference : https://www.englishclub.com/english-for-work/medical-specialists.htm , consulted on july 08th,2019