Lesions can occur in any area of the body consisting of soft tissue, but are most commonly found on the skin or in the mouth. There are many different circumstances that can induce the formation of lesions.
Lesions are caused by any process that damages tissues. A cancerous tumor along with surrounding tissue damaged by the tumor is categorized as an example of lesion. Trauma, including electrocution and chemical burns can also cause lesions. Certain diseases produce lesions,like chicken pox produce some skin deformities. Lesions can also be caused by metabolic processes, like an ulcer, arthritis(simply to say, joint damages).
Lesions are sometimes intentionally inflicted during neurosurgery, to treat epilepsy and other brain disorders.
Note that lesions are not limited to animals or humans; damaged plants are said to have lesions.