These are some cold hard facts about the common cold. Although colds don’t kill like cancer, they take a significant toll every year in the form of millions of lost work hours and billions of dollars spent on medications. The most prevalent cold is caused by rhinovirus, a family of more than 100 viruses. Gwaltney estimates that the various forms of rhinovirus are responsible for at least half of all colds. Although rhinoviruses are all related, each contains just enough different genetic material to make it impossible for the body to develop blanket immunity-or for researchers so far to develop a vaccine.