pes·ti·lent 
(pĕstə-lənt)
(pĕstə-lənt)adj.
1.
a. Causing or likely to cause epidemic disease: a pestilent swamp.
b. Infectious or epidemic: a pestilent disease.
c. Archaic Tending to cause death; deadly.
2. Morally or socially harmful; pernicious: pestilent writings.
3. Causing annoyance or disapproval: a pestilent fool.
[Middle English, from Old French, from Latin pestilēns, pestilent-, from pestis, pestilence; see PEST.]