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A contagious disease is categorized as virulent when:
- it is easily transmitted from person to person through casual contact;
- it does not require direct contact from patient to patient in order to be transmitted;
- it can live outside a host patient for considerable periods of time and still remain viable; and
- it causes the disease in almost every patient who is exposed to it unless they are already immune to it.
In short, a virulent disease is very easy to catch. Virulent contagious diseases are far more likely to result in epidemics.
The virulence of a disease is not related to its mortality rate. For example, both the common cold and smallpox are virulent, but one is trivial while the other is likely to be fatal. Conversely, many fatal diseases, such as HIV, and tuberculosis are not very virulent.