A pest is an organism that people see as damaging or unwanted, as it harms agriculture through feeding on crops in the field and stored grain products. An animal can also be a pest when it causes damage to a wild ecosystem or growing crops. The term pest is used to refer specifically to harmful animals but it also relates to all other harmful organisms, including nematodes, fungi and viruses. However, these three are considered as pathogens causing diseases than being as pests. Many weeds (plant pests) are also seen as useful under certain conditions when growing in the right place and time.