Respiratory viruses are both unpredictable and commonplace. The name of the most well-known one, Influenza, originated in 15th century Italy, and comes from the old Italian expression influenza dei pianeti or influence of the planets. They could not explain its sudden and unaccountable behavior and ascribed its capricious nature to the influence of planets. However, influenza is just one of the many agents involved in active respiratory infections; there are scores of known ones which give a spectrum of clinical presentations, from a mild cold to severe pneumonia. We have no idea how many agents there are. Since 1970, 1,500 pathogens have been discovered – 70% have come from animals. Some authors report that up to 40% of respiratory infections have no recognised causes.