Influenza is also known as flue, or grippe and it is an infectious respiratory disease. Flu is caused by the influenza virus which, when inhaled, attacks cells in the respiratory tract. The most often visible flu symptoms are fatigue, a hacking cough, fever and chills sometimes accompanied by muscle aches. Influenza may also contribute to the development of secondary, life-threatening infections. Although influenza virus hardly ever causes gastrointestinal symptoms it is frequently blamed for intestinal or stomach flu.

Influenza is a lot more serious than the common flu as the infection may spread very rapidly and outbreak suddenly. Only in the United States there are about 20,000 deaths attributed to influenza every year. The first descriptions of influenza were written over 2000 years ago in the ancient Greece. Nowadays, we distinguish three types of influenza viruses, identified as A, B, and C. Influenza A can infect not only humans, but also birds, horses and pigs, whereas only humans can be infected by influenza virus type B and C. Influenza A is responsible for the vast majority of all flu cases, while infection with influenza virus type B and C are much less common and causes milder illnesses.