In pretty much every case, saying "the injured" will refer to more than one person being injured in some kind of accident.
A bus crashed on the motorway today. The injured were taken to a local hospital.
A bus crashed on the motorway today. One person was slightly injured and was taken to hospital.
One person is never referred to as "the injured", he/she would always be called "the injured man/woman/child" etc.
Don't ask me why. It just is!
Student or Learner