Measles virus is an enveloped, negative-sense, single-stranded RNA virus belonging to the genus Morbillivirus within the family Paramyxoviridae. It is one of the most contagious human pathogens known, with a basic reproduction number estimated at 12 to 18 in unvaccinated populations. Measles virus is antigenically stable, with only a single serotype recognised, meaning that immunity acquired through infection or vaccination is broadly protective and long-lasting.

Measles virus enters host cells through the interaction of its haemagglutinin (H) surface glycoprotein with one of two principal receptors: CD150 (also known as SLAM), which is expressed on immune cells including macrophages, dendritic cells, and activated lymphocytes, and nectin-4 (PVRL4), expressed on epithelial cells of the respiratory tract. Receptor binding triggers conformational changes in the fusion (F) glycoprotein, leading to direct fusion of the viral envelope with the host-cell plasma membrane and release of the viral ribonucleoprotein complex into the cytoplasm. Replication occurs entirely in the cytoplasm, driven by the viral RNA-dependent RNA polymerase. Progeny virions bud from the apical surface of epithelial cells or, in immune cells, may spread directly from cell to cell without extracellular release.

The measles virus genome is non-segmented and encodes six structural proteins: the nucleoprotein (N), phosphoprotein (P), matrix protein (M), fusion glycoprotein (F), haemagglutinin (H), and large protein (L, the catalytic subunit of the viral polymerase). Two additional non-structural proteins, V and C, are produced from the P gene through RNA editing and alternative translation initiation, and both contribute to evasion of host innate immune responses, in particular by antagonising interferon signalling. The H protein mediates receptor binding and is the primary target of virus-neutralising antibodies, while the F protein is required for membrane fusion and is also targeted by the host immune response.

Measles virus is transmitted through the respiratory route, primarily via airborne droplet nuclei that can remain suspended in air and infectious for up to two hours after an infected person has left a room, a feature that contributes substantially to its high transmissibility. Initial infection occurs in the respiratory epithelium, from where the virus rapidly spreads to regional lymph nodes and then via a primary viraemia to lymphoid tissues throughout the body, causing widespread immune cell infection before a secondary viraemia seeds the respiratory epithelium, skin, and other organs. The characteristic maculopapular rash appears approximately 14 days after exposure and coincides with the onset of the adaptive immune response.

The clinical course of measles typically begins with a few days of fever, cough, a runny nose, and red eyes, and one early telltale sign is the appearance of tiny white spots on the buccal mucosa, known as Koplik spots. The generalised rash then appears and spreads from the face downwards over the body. In immunocompetent individuals the illness resolves within one to two weeks, but measles causes a profound and prolonged immunosuppression that can persist for weeks to months after acute infection, increasing susceptibility to secondary bacterial and viral infections. Serious complications include pneumonia, the leading cause of measles-related death, croup, otitis media, and encephalitis. A rare but uniformly fatal late complication, subacute sclerosing panencephalitis (SSPE), can develop years after measles infection, particularly in children infected before the age of two.

Despite the availability of a safe, effective, and inexpensive vaccine, measles remains a significant global public health burden. Worldwide, measles causes hundreds of thousands of deaths annually, the vast majority in children under five in low-income countries with suboptimal vaccine coverage. Resurgences of measles in countries with previously high vaccination coverage have also highlighted the vulnerability created by declining vaccine uptake. Achieving and sustaining the greater than 95% population-level immunity required to interrupt measles transmission demands two doses of a measles-containing vaccine, most commonly delivered as the combined measles-mumps-rubella (MMR) or measles-mumps-rubella-varicella (MMRV) vaccine.

There is no specific licensed antiviral treatment for measles, and clinical management is primarily supportive, including fever management, hydration, nutritional support, and prompt treatment of secondary bacterial infections. High-dose vitamin A supplementation is recommended by the World Health Organization for all children with measles in regions where vitamin A deficiency is prevalent, and has been shown to reduce the severity and mortality associated with the disease. Vaccination remains the cornerstone of measles prevention and control, and global elimination targets continue to drive efforts to improve vaccine delivery, strengthen surveillance, and reach under-immunised populations.

Overall, measles virus is a highly infectious and immunomodulatory pathogen whose continued circulation represents a preventable burden of morbidity and mortality worldwide. Its remarkable stability as a single serotype, combined with the existence of a highly effective vaccine, makes measles a candidate for eventual global eradication, yet achieving and sustaining the necessary vaccine coverage remains a substantial challenge for national and international public health programmes.