epidemicity

noun

ep·​i·​de·​mic·​i·​ty ˌep-ə-ˌdem-ˈis-ət-ē  ˌep-əd-ə-ˈmis- 

plural epidemicities

: the quality or state of being epidemic

specifically : the relative ability to spread from one host to others

the epidemicity of typhoid bacteria