The flu (also known as influenza) is an infectious disease caused by a virus – the influenza virus. While most people confuse the flu with the common cold (also an infectious disease, but caused mainly by rhinovirues), the symptoms can be more severe. In fact, the flu can be deadly, especially mutated versions, such as the one that caused the Spanish flu in 1918, which killed around 20-100 million people.

The flu (here we refer to influenza type A only) is categorized by two viral proteins, the haemagglutinin and the Neuraminidase. Due to the expression of different antigens, a virus is called e.g. H1N1 (the one from the Spanish flu), or H3N2 the one that is currently active in Germany.

By the way, Google published a paper in Nature in 2009 in which they demonstrated the use of Google search for detecting influenza epidemics. The results can be found in Google Flu.