A pest is an animal which has characteristics which people regard as injurious or unwanted. This is most often because it causes damage to agriculture through feeding on crops or parasitising livestock, such as codling moth on apples, or boll weevil on cotton. An animal can also be a pest when it causes damage to a wild ecosystem or carries germs within human habitats. Examples of these include those organisms which vector human disease, such as rats and fleas which carry the plague disease, or mosquitoes which vector malaria.

Plant pests, and sometimes fungal pests, are usually called weeds.

It is possible for an animal to be a pest in one setting but beneficial or domesticated in another (for example, European rabbits introduced to Australia caused ecological damage beyond the scale they inflicted in their natural habitat). Many weeds are also seen as useful under certain conditions, for instance Paterson's curse is often valued as food for bees and as a wildflower, even though it can poison livestock.

The concept of a pest is anthropogenic, based on human purposes.

Related is pestilence, which is any highly-infectious (epidemic) disease.

See also

• biological pest control
• pesticide.

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