Measles (Rubeola) is classically identified by the presence of Koplik spots, which are small, irregular, bluish-white lesions on the buccal mucosa opposite the molars. These spots appear 1-2 days before the characteristic measles rash and serve as a pathognomonic sign. Rubella, Varicella-Zoster, and Herpes Simplex viruses do not produce Koplik spots. This makes Koplik spots a key early diagnostic feature specific to measles infection.

Reference: Harrison's Principles of Internal Medicine, 20th Edition, Volume 2, Infectious Diseases, Chapter 157, p. 1139