 A flu-like syndrome can also be caused by an influenza vaccine or other vaccines, and by opioid withdrawal in physically dependent individuals.Influenza-like illness is a nonspecific respiratory illness characterized by fever, fatigue, cough, and other symptoms that stop within a few days.Most cases of ILI are caused not by influenza but by other viruses (e.g., rhinoviruses, coronaviruses, human respiratory syncytial virus, adenoviruses, and human parainfluenza viruses).