Measles has been recognized as a distinct clinical disease for more than 10 centuries and in the developing world is associated with high mortality rates in early childhood.The epidemiology of measles is markedly affected by population size, density, movement, and social behavior. In the absence of vaccination, the disease infects essentially everyone at some time during life except in isolated populations. Beginning in 1963 the availability and increasing use of live attenuated measles vaccines have made prevention possible. Countries in the Americas, Europe, and the Eastern Mediterranean have undertaken the interruption of measles transmission.1,2 Measles has not been endemic in the United States since 1997.3 Indigenous transmission of measles was interrupted in the Americas in 2002.4
Measles is one of the most contagious of infectious diseases. Mathematical models suggest that in a totally susceptible population the average case of measles may result in transmission of measles to 12–18 persons.5 Thus it is estimated that the immunity level needed to interrupt transmission is on the order of 94% or higher. Contact rates vary substantially by age group and affect the age-specific level of immunity needed to prevent transmission. Although high levels of immunity substantially reduce the likelihood that susceptible persons within a population will be exposed to disease, there is no level of immunity short of 100% that will absolutely guarantee absence of transmission.
Following an incubation period averaging 10–12 days (range 8–16 days), the patient typically has fever and malaise, followed shortly thereafter by cough, coryza, and conjunctivitis.6 An enanthem, characterized by small bluish white spots on a red background (Koplik's spots), may be seen on the buccal mucosa within the two days before and after the onset of rash. The characteristic maculopapular rash of measles usually appears an average of 14 days after infection begins and typically 2–4 days after the onset of the prodromal symptoms. The exanthem classically starts on the face and hairline and then spreads to the trunk and extremities. The patient's temperature usually peaks 1–3 days following the onset of rash. The rash, areas of which fade in order of appearance, typically lasts 5–7 days, and the illness is entirely gone by 10–14 days after the onset of symptoms. There are few clinically inapparent primary infections.
The patient is infectious during the prodromal period and for the first few days of rash. The infectious period is usually considered to stretch from four days before to four days after the onset of rash. Measles is usually transmitted in large respiratory droplets, requiring close contact between patients and susceptible persons. However, measles virus can survive for at least two hours in fine droplets, and airborne spread has been documented.7,8 Neither a long-term infectious carrier state nor an animal reservoir is known.