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.