Atypical measles arises when children or young adults previously vaccinated with killed measles vaccine are exposed to the wild type of measles virus during a community outbreak and subsequently develop disease. This inactivated vaccine, produced in the 1960s, was ineffective in that it failed to produce long-term protective antibody formation, with antibody levels decreasing within months to a couple of years following vaccination. This particular measles vaccine's use was terminated in 1967. Atypical measles has also been described in patients who have had live measles vaccine.

Patients with atypical measles typically develop a prodrome characterized by high spiking fever (up to 40°C [104°F]). This is accompanied by myalgia, cough, and headache. The polymorphous rash that develops is erythematous, macular or papular, and begins on the extremities and spreads to involve the trunk. The lesions then become vesicular, petechial, or purpuric. The exanthema generally occurs on the third to fourth day of the illness. Prostration, headache (which may be occipital), nausea, and vomiting may be followed by a sore throat, conjunctivitis, and photophobia. Cough, occasionally with pleuritic pain, is almost universal. A severe pneumonitis occurs in most patients and may be life-threatening. The illness appears to be self-limiting, lasting from 3 to 10 days. Additional clinical features may include abdominal pain and hepatic dysfunction.

Hospitalization should be considered. Other complicating features of the disease include otitis media, pneumonia, acute thrombocytopenic purpura, and encephalitis (2–7 days after the start of the rash). A late and rare complication is subacute sclerosing panencephalitis, which presents with mental deterioration, behavioral changes, and myoclonic jerks.