A pest is an animal or tiny organism that causes harm to humans, animals, food, or living conditions.
Pests are damaging because they attack and eat vegetables and other crops, cause and spread disease, or damage stored food.
Pests include weeds, and animals such as rats. Some pests are parasites which live on or inside animals, and are very small.
Pests and disease
The smallest pests are pathogens, which can produce disease. Examples of pathogens are bacteria, fungi and viruses.
Pests typically reproduce very quickly so their population can increase rapidly, leading to infestations which destroy entire crops and cause illnesses, or death, in animals.
Some of these diseases can be passed from animals to humans.
Protecting crops and animals
Both crops and animals need to be protected from the wide variety of different pests and organisms that present a threat.
A pest is not always a pest
It is possible for an animal to be a pest in one place and beneficial or accepted in another. Weeds in some countries are seen as useful in others.