There are three major groups of these viruses, designated A, B, and C. Major influenza epidemics are usually caused by a strain of the A virus. There may also be vomiting and an aversion to light and noise. As the illness progresses, the cough may become less dry and painful because of the production of sputum. If no complications develop, the fever generally lasts for about five days. Recovery is usually rapid and without relapse, although it may be accompanied by some weakness and depression.
Influenza lowers the body's resistance to infection.
This makes the patient vulnerable to invasion by other organisms that may cause secondary infections, especially of the lungs and bronchial tubes, sinuses, and ears, causing such conditions as pneumonia, bronchitis, sinusitis, and otitis media. Pneumonia is by far the most serious complication, since it is often fatal in elderly people and in patients with heart and lung diseases or malignancies. Secondary pneumonia is the major cause of death related to influenza in economically developed countries. The patient should go to bed as soon as symptoms appear and should remain there until a complete recovery has been made. The patient should drink plenty of fluids, especially while there is a fever.