Why do we get more colds and flu in winter than in summer? And why do they increase when the weather turns cold? And why are colds and flu more common in the Tropics than in the Europe and the USA in the summer? Respiratory viruses need to keep out of the lungs, heart, brain etc of their host, or they will stop us from moving around. This will reduce spreading. Could the natural temperature-sensitivity of virtually all respiratory viruses be the key to understanding all of this?