Virulent, a.
Etymology: [L. virulentus, fr. virus poison: cf. F. virulent. See Virus.]
1. Extremely poisonous or venomous; very active in doing injury. A contagious disorder rendered more virulent by uncleanness. Sir W. Scott.
2. Very bitter in enmity; actuated by a desire to injure; malignant; as, a virulent invective.
Shades of meaning
Fernald has no entry of its own for virulent; he lists it under bitter.
He lists it without comment; the entries above explain its neighbors.