When something is pestilent, it's contagious and often deadly. At one time, polio was considered to be a pestilent disease in North America.

Something harmful that spreads — whether it's an illness or violence or a really bad idea — can be described with the adjective pestilent. The rise of poverty in some places, or the spread of chicken pox among preschoolers, are both pestilent. The Latin origin is pestilentem, which comes from pestilis, "of the nature of a plague," with its root of pestis, "deadly contagious disease."

Definitions of pestilent
  1. adjective
    likely to spread and cause an epidemic disease
    synonyms: pestiferous, pestilential, plaguey
    (especially of medicine) of disease or anything resembling a disease; attacking or affecting many individuals in a community or a population simultaneously
  2. adjective
    exceedingly harmful
    synonyms: baneful, deadly, pernicious
    injurious to physical or mental health
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