Infectious, plague-causing, and definitely disease-bearing: If it's likely to bring along pestilence, then it's pestilential.

Today we know that dirty hands, unclean water, and bubonic plague-carrying rats can all be potentially pestilential. But back in the olden days, the culprit wasn't so clear. Too much merriment, too little merriment, too much fresh air, too little fresh air — all were considered pestilential at one time or another.

Definitions of pestilential
  1. adjective
    likely to spread and cause an epidemic disease
    “"a pestilential malignancy in the air"- Jonathan Swift”
    synonyms: pestiferous, pestilent, plaguey
    (especially of medicine) of disease or anything resembling a disease; attacking or affecting many individuals in a community or a population simultaneously
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