1660s (virulency attested from 1610s), from Latin virulentia, from virulentus (see virulent).
virulence vir·u·lence (vēr'yə-ləns, vēr'ə-)n.
The quality of being poisonous.
The capacity of a microorganism to cause disease.
The capacity of a pathogen, such as a microorganism or toxin, to produce disease.
Note: “Virulent” substances or organisms are extremely pathogenic or toxic.