evidence: AboutSigns and SymptomsCauses and SpreadFlu Complications and People at Higher RiskPreventionVaccine BasicsDiagnosisFlu TreatmentView All searchclose search [...] Flu is a contagious respiratory illness that infect the nose, throat, and sometimes the lungs. [...] Influenza (flu) is a contagious respiratory illness caused by influenza viruses that infect the nose, throat, and lungs.