Atypical measles occurs in individuals who were previously immunized with the killed measles vaccine between 1963 and 1967 and who have incomplete immunity. When they are exposed to the measles virus, a mild or nonexistent prodrome of fever, headache, abdominal pain, and myalgias precedes a rash that begins on the hands and feet and spreads centrally. The rash is most prominent in the body creases and may be macular, hemorrhagic vesicles, petechial, or urticarial. All persons vaccinated after 1967 received the live attenuated measles vaccine, which is not associated with atypical measles.

Scarlet fever is characterized by prodrome of sore throat, headache, vomiting, abdominal pain and fever followed by appearance of rash 1-2 days after onset of illness, first on the neck and then extending to the trunk and extremities. The exanthem is usually coarse and the erythema blanches with pressure. A few days following generalization of the rash, it becomes more intense along skin folds and produces lines of confluent petechiae known as the Pastia sign. These lines are caused by increased capillary fragility. The rash begins to fade 3-4 days after onset, and the desquamation phase begins. This phase begins with flakes peeling from the face. Peeling from the palms and around the fingers occurs about a week later and can last up to a month. Diagnosis of scarlet fever can be established by throat culture for streptococcus and ASLO test.