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.

Some contagious diseases effectively have a rate approaching 100%. This includes diseases where exposure to infection is quite easy (such as the common cold, influenza, and smallpox, all of which can be transmitted through the air by breathing in the pathogen), to those where it is quite difficult to be exposed (such as HIV and Hepatitis C, which are very difficult to contract).

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%.

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