Flu is an illness which is similar to a bad cold but more serious. It often makes you feel very weak and makes your muscles hurt. I got flu He had come down with the flu. a common illness that makes you feel very tired and weak, gives you a sore throat, and makes you cough and have to clear your nose a lot = influenza (influenza)
{i} influenza, contagious viral disease characterized by fever and inflammation of the respiratory tract (Medicine)