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Marek's disease is a viral disease caused by the Marek's disease virus, which primarily affects chickens. The most significant lesions associated with Marek's disease are nerve enlargements, which occur due to the infiltration of lymphoid cells in the nervous system. This infiltration can lead to various neurological signs, such as paralysis or incoordination, which can be observed in affected birds.

In addition to nerve enlargement, Marek's disease can also result in the development of tumors and lesions in various organs, including the liver, spleen, and skin, but the hallmark lesion remains the enlargement of peripheral nerves. This enlargement can be detected during necropsy and is a definitive indicator of the disease.

While other lesions might be associated with different diseases or conditions affecting birds, such as facial swelling and sinusitis, hemorrhagic skin lesions, or wrinkled eggs, these are not characteristic of Marek's disease. Therefore, the occurrence of nerve enlargement is the primary lesion that confirms an infection with Marek's disease virus, making it the most commonly associated lesion in the context of this disease.