Complex sentence: Some researchers believe that atherosclerosis may be caused by an infection of the vascular smooth muscle cells; chickens, for example, develop atherosclerosis when infected with the Marek's disease herpesvirus.
Split into simpler sentences: Some researchers believe that atherosclerosis may be caused by an infection of the vascular smooth muscle cells. Chickens, for example, develop atherosclerosis when infected with the Marek's disease herpesvirus.