The infection rate for a contagious illness describes what percentage of succeptible patients (i.e. those who are not immune to it from previously having the illness or vaccination) exposed to the disease pathogen in appropriate circumstances will actually develop symptoms of the disease. This rate is independent of how contagious or virulent the pathogen is. However, other contagious diseases, even quite serious ones, do not infect a large percentage of the people who are exposed to them. For example, the infection rates for leprosy and tuberculosis are both well under 10%.