This document discusses ulcerative and inflammatory lesions of the oral cavity. It focuses on aphthous ulcers (canker sores), herpesvirus infections, oral candidiasis, and lesions seen in AIDS patients such as Kaposi's sarcoma. Aphthous ulcers are common, painful ulcers caused by various triggers that typically resolve within a few weeks. Herpesvirus infections cause cold sores and fever blisters via virus reactivation. Oral candidiasis (thrush) afflicts those with weakened immunity. Lesions in AIDS patients include opportunistic infections and Kaposi's sarcoma.