A pest is an organism which has characteristics that are regarded by humans 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.
The term pest may be used to refer specifically to harmful animals but is also often taken to mean all harmful organisms including fungi and viruses. Pesticides are chemicals that are used to control or protect other organisms from pests.
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 Patterson's curse is often valued as food for honeybees and as a wildflower, even though it can poison livestock.
The concept of a pest is anthropogenic, based on human purposes and perceptions.