Influenza, specific acute respiratory disease caused by viruses and characterized by fever, head cold, cough, headache, malaise, and inflamed mucous membranes of the respiratory tract. It usually occurs as an epidemic in the winter. Hemorrhage, bronchitis, pneumonia, and sometimes death occur in severe cases. Acute epidemics occur about every 3 years. Persons at high risk of developing severe diseases are those with chronic lung disease, heart valve disease, or congestive heart failure; pregnant women in the third trimester; and elderly persons who are confined to bed. The antiviral drug amantadine has a beneficial effect on fever and respiratory symptoms if given early in uncomplicated influenza. It is of no benefit when the illness is complicated with pneumonia, but might improve recovery from a lung infection. The basic treatment otherwise is the relief of symptoms. Aspirin, paracetamol, and other drugs to lower the temperature and relieve pain are helpful. Vaccines that include the prevalent strains of influenza viruses have a 60% chance of reducing the incidence of infection for 1 to 2 years after vaccination. The immunity is less when the virus changes appreciably (antigenic drift) and when a major viral mutation occurs (antigenic shift). No significant protection is afforded unless the new strain is incorporated into the vaccine. Vaccination is especially important for the aged and for those with heart, lung, or other chronic diseases.