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.
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.