The onset of influenza is usually sudden. Symptoms are readily apparent and rather severe, especially fever, which rises rapidly to high body temperatures. The first symptoms appear immediately after infection and generally subside within a few days. Influenza symptoms are “a-specific”, which means that they are common to many other diseases. Some of them are very similar to those of the common cold and at times it may be difficult to tell them apart.

The most common influenza symptoms are:

• Fever: which always occurs and rises quickly to rather high temperatures (even above 40°Centigrade/104˚ Fahrenheit ). Usually a fever lasts three to four days.

• Headache: a typical symptom. Generally pain is intense and sudden.

• Fatigue and general weakness: typical influenza symptoms. Those symptoms can last for days after recovery.

• Pain in muscles and bones: equally typical of influenza. They may be rather severe.

• Sore throat: may or may not occur.

• Coughing: a frequent symptom, usually dry and persistent.

• Inflammation of the respiratory tract: the most common being laryngitis and bronchitis.

Also forms of intestinal flu are sometimes mentioned, but actually there are no influenza viruses causing intestinal symptoms. These symptoms occur in some cases but they are due to other infections.