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."