Definition and description of Flu : Flu is referred to as Influenza in medical terminology. Flu is a highly contagious and acute respiratory tract infection that normally occurs in winter season. Generally, flu appears in form of epidemic. Though flu can affect people of all ages, children are affected more. Immuno-suppressed people, young children and people with chronic diseases experience the high severity of flu. Pneumonia (viral pneumonia or bacterial infection) is most common complication produced by flu. Other complications include chronic obstructive pulmonary disease, myositis, myocarditis, Reye’s syndrome, encephalitis and pericarditis.