Measles is a highly contagious acute viral infection caused by measles virus, a member of the genus Morbillivirus in the Paramyxoviridae family. It primarily affects the respiratory system initially and then produces a characteristic systemic illness involving fever and a widespread maculopapular rash. Measles is among the most transmissible human infections because infected people can release virus into shared air and susceptible individuals can become infected after relatively brief exposure. Infection usually produces long-lasting immunity, while vaccination provides highly effective prevention.
The virus enters through the respiratory tract or conjunctival surfaces and replicates initially in local tissues before spreading through the body. After an incubation period, infected people develop a prodromal illness typically characterised by fever, cough, coryza and conjunctivitis. Koplik spots, small lesions on the oral mucosa, are a classic clinical finding but are not always observed. The characteristic rash generally begins on the face or hairline and progresses downward over the body. Infectiousness begins before the rash appears, which contributes substantially to transmission before diagnosis.
Measles can cause complications affecting multiple organ systems. Common or important complications include otitis media, diarrhoea and pneumonia, while encephalitis is a serious neurological complication. Severe disease is more likely in young children, adults, pregnant people and individuals with impaired immunity, although serious complications can occur in other groups. Measles can also cause transient suppression of immune memory, sometimes described as immune amnesia, which can increase susceptibility to other infections after recovery.
A particularly serious delayed complication is subacute sclerosing panencephalitis, a rare progressive neurological disease caused by persistent measles virus infection in the central nervous system. It usually develops years after the original measles infection and can cause cognitive decline, behavioural changes, seizures, movement abnormalities and progressive neurological deterioration. The complication is uncommon, but its existence illustrates that recovery from the acute rash illness does not eliminate every possible long-term consequence.
Diagnosis is based on clinical presentation together with laboratory confirmation when appropriate, particularly during outbreaks or when elimination status is important. Molecular detection and serological testing can help establish infection. Because measles is highly contagious, suspected cases require prompt public-health attention, appropriate infection-control measures and assessment of contacts. Exposure management can involve vaccination or immunoglobulin in selected circumstances, depending on timing, age, immune status and clinical guidance.
The principal preventive measure is vaccination with measles-containing vaccine, usually administered as part of a combined measles, mumps and rubella vaccine. High population coverage is required to maintain community protection because measles spreads efficiently and susceptible clusters can permit outbreaks. Two appropriately timed vaccine doses provide strong protection for most people. Maintaining vaccination coverage is particularly important because measles can be reintroduced through travel into populations where local transmission has previously been interrupted.
Measles is not the same condition as German measles, which is rubella, despite historical terminology sometimes causing confusion. Measles also differs from chickenpox because the pathogens, rash characteristics, complications and prevention strategies are different. Antibiotics do not cure measles because its causative agent is a virus, although bacterial complications may require separate treatment. Another misconception is that measles is harmless because most uncomplicated cases resolve; in reality, serious complications and deaths can occur, especially where vaccination and supportive healthcare are limited.
The history of measles vaccination demonstrates the effect of immunisation on infectious-disease incidence. Before widespread vaccination, measles caused large numbers of childhood infections, whereas sustained high vaccine coverage can interrupt endemic transmission. Elimination requires exceptionally high immunity because the virus is so contagious. Surveillance, rapid case investigation, laboratory confirmation, contact tracing and vaccination programmes therefore remain important even in countries where measles transmission has previously been reduced substantially.
