The Collaborative International Dictionary of English v.0.48:

Infest \In*fest"\, v. t. [imp. & p. p. Infested; p. pr. & vb.
   n. Infesting.] [L. infestare, fr. infestus disturbed,
   hostile, troublesome; in in, against + the root of defendere:
   cf. F. infester. See Defend.]
   To trouble greatly by numbers or by frequency of presence; to
   disturb; to annoy; to frequent and molest or harass; as,
   fleas infest dogs and cats; a sea infested with pirates.
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         To poison vermin that infest his plants. --Cowper.
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         These, said the genius, are envy, avarice,
         superstition, love, with the like cares and passions
         that infest human life.                  --Addison.
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         And the cares, that infest the day,
         Shall fold their tents, like the Arabs,
         And as silently steal away.              --Longfellow.
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The Collaborative International Dictionary of English v.0.48:

Infest \In*fest"\, a. [L. infestus. See Infest, v. t.]
   Mischievous; hurtful; harassing. [Obs.] --Spenser.
   [1913 Webster]

WordNet (r) 3.0 (2006):

infest
    v 1: invade in great numbers; "the roaches infested our kitchen"
         [syn: infest, overrun]
    2: occupy in large numbers or live on a host; "the Kudzu plant
       infests much of the South and is spreading to the North"
       [syn: invade, overrun, infest]
    3: live on or in a host, as of parasites

Moby Thesaurus II by Grady Ward, 1.0:

18 Moby Thesaurus words for "infest":
   beset, crawl with, creep with, flood, infiltrate, inundate, invade,
   overrun, overspread, overswarm, penetrate, permeate, pervade,
   plague, ravage, swarm, swarm over, swarm with