It used to be thought that a pathogen (or germ) and its host develop in a 'mutually benign relationship'—this was called the 'Law of Declining Virulence'. If the common cold killed us there would be less hosts and the virus would decrease. So the common cold virus mutated to become less lethal, and more common. The Law of Declining Virulence was debunked in the 1980s, and the pathogen/host relationship is actually pretty complicated.