Measles

Virus

Enveloped, negative-sense ssRNA, morbillivirus (Paramyxoviridae). AMONG MOST CONTAGIOUS human pathogens — R0 ~12-18 (>> flu, >SARS-CoV-2). Herd immunity threshold ~95% — much higher than most vaccine-preventable diseases.

Pathogenesis

Respiratory droplet + AIRBORNE (viable in air up to ~2hr after infected person leaves room) — Airborne Precautions category (with TB, varicella, see Standard Precautions topic), not just Droplet.

Primary+secondary viremia (see General Virus Properties) → skin, respiratory tract, other organs. NOTABLE: profound TRANSIENT IMMUNOSUPPRESSION during/after infection — “IMMUNE AMNESIA” (depletes memory cells, erodes prior immunological memory to unrelated pathogens). Substantial underappreciated mortality contributor (vulnerability extends beyond acute illness).

Clinical Manifestations

Prodrome: fever + “3 C’s” (cough, coryza, conjunctivitis). KOPLIK SPOTS: white/blue-grey spots, erythematous base, buccal mucosa, 1-2 days BEFORE rash. PATHOGNOMONIC (no other common childhood exanthem has this enanthem). Exanthem: erythematous maculopapular, CEPHALOCAUDAL spread (face/hairline→trunk→extremities), fever peaks at max rash extent.

Complications (drive real morbidity/mortality, not the exanthem itself):

Otitis media — COMMONEST complication
Pneumonia (viral or 2° bacterial, exploits immune amnesia) — LEADING mortality cause
Acute encephalitis — ~1/1000 cases, substantial mortality/morbidity

SSPE (Subacute Sclerosing Panencephalitis): separate mention — SLOW VIRUS infection (see CNS topic, with prion disease). RARE, DELAYED (7-10yr post-acute infection), persistent defective virus in CNS, progressive decline+myoclonus+death, UNIFORMLY FATAL. Risk HIGHER if original infection at VERY YOUNG AGE (<2yr) — specific epidemiological point.

Laboratory Diagnosis

Serology (IgM, positive from rash onset) = standard method. RT-PCR (respiratory specimen/urine): direct detection + genotyping (outbreak tracing, distinguishes wild-type from vaccine-strain in recently vaccinated + compatible rash). Often CLINICALLY diagnosed (distinctive picture) + compatible epi context — lab confirms for public health reporting, not usually needed to establish diagnosis from uncertainty.

Treatment

NO specific antiviral. Entirely SUPPORTIVE. VITAMIN A supplementation: specific evidence-based intervention, recommended ALL children with acute measles (esp. resource-limited/vitamin A insufficient regions) — reduces morbidity/mortality (epithelial/immune function role).

Prevention — Vaccination

Live attenuated MMR (combines measles+mumps+rubella). NIS: dose 1 ~9-12mo, dose 2 early childhood (see Vaccines topic).

Given exceptionally high R0/herd threshold: ANY MEANINGFUL COVERAGE GAP → RAPID outbreak risk — real pattern in vaccine-hesitancy outbreaks globally, practical illustration of herd immunity as operational requirement (not just abstract concept) for this-transmissible pathogen.

PEP: vaccine (<72hr post-exposure) or measles immunoglobulin (if can’t safely get live vaccine — young infants, pregnant, immunocompromised) — prevents/attenuates disease in susceptible contact.

The Virus

Measles virus is an enveloped, negative-sense ssRNA morbillivirus (a genus within the Paramyxoviridae, sharing the family’s general enveloped, helical-symmetry, negative-sense replication pattern covered under General Properties of Viruses) — genuinely notable for being among the most contagious human pathogens known, with a basic reproduction number (R₀) commonly cited in the range of 12–18 (meaning a single case, in a fully susceptible population, would be expected to generate that many secondary cases) — a figure dramatically higher than influenza’s or even SARS-CoV-2’s, and the underlying reason population vaccine coverage needs to be exceptionally high (generally cited around 95%) to achieve herd immunity against measles specifically, well above the threshold needed for most other vaccine-preventable diseases.

Pathogenesis

Transmission is by respiratory droplet and, genuinely notably, by airborne spread — measles virus remains viable and infectious in air for up to about 2 hours after an infected person has left a room, a real, distinctive feature placing it alongside TB and varicella under Airborne Precautions (see Needle Stick Injury and Standard Precautions) rather than the more commonly encountered Droplet Precautions category most respiratory viruses fall under. After initial respiratory tract infection, the virus disseminates via a primary and secondary viremia (the same two-wave viremic pattern covered generally under General Properties of Viruses) to skin, respiratory tract, and other organs — and measles is genuinely notable among viral infections for causing a profound, transient immunosuppression during and after acute infection, from measles-driven depletion of immune memory cells (a phenomenon termed “immune amnesia,” where previously acquired immunological memory against unrelated pathogens is measurably eroded) — this immunosuppression is a substantial, underappreciated contributor to measles-associated mortality, since it leaves the patient vulnerable to secondary bacterial and other infections for a period extending well beyond the acute measles illness itself.

Clinical Manifestations

The classic course runs through a prodrome (fever, and the “3 C’s” — cough, coryza, conjunctivitis) followed by the pathognomonic Koplik spots (small, white/blue-grey spots on an erythematous base on the buccal mucosa, appearing 1–2 days before the skin rash and genuinely diagnostic when seen, since no other common childhood exanthem produces this specific enanthem) and then the exanthem itself — an erythematous, maculopapular rash beginning on the face/hairline and spreading cephalocaudally (head-to-trunk-to-extremities, in that specific downward-progressing order) over several days, with fever typically peaking as the rash reaches its most extensive distribution.

Complications are genuinely common and are what actually drive measles’ significant morbidity/mortality burden, rather than the acute exanthem itself: otitis media (the commonest complication overall), pneumonia (either primary viral pneumonia or secondary bacterial pneumonia exploiting the immune-amnesia-driven vulnerability described above, and the leading cause of measles mortality), and acute encephalitis (occurring in roughly 1 in 1,000 cases, with substantial associated mortality/neurological morbidity in survivors). Subacute sclerosing panencephalitis (SSPE) deserves specific, separate mention as a genuinely distinctive, uniformly fatal slow virus infection (covered under the slow-virus-disease category alongside prion disease in Central Nervous System Infections) — a rare, delayed complication occurring years (typically 7–10 years) after the original acute measles infection, from persistent, defective measles virus surviving in CNS tissue, producing progressive neurological decline, myoclonus, and eventual death; SSPE risk is genuinely notable for being disproportionately higher when the original measles infection occurs at a very young age (under 2 years), a real, specific epidemiological point.

Laboratory Diagnosis

Serology — IgM antibody detection, positive from around the time the rash appears — is the standard diagnostic method in most clinical settings. RT-PCR on a respiratory specimen or urine offers direct viral detection and genotyping (relevant to outbreak source-tracing and distinguishing wild-type from vaccine-strain virus in a recently vaccinated individual with a compatible rash). In practice, given measles’ genuinely distinctive clinical picture (prodrome, Koplik spots, cephalocaudal rash), diagnosis is often made substantially on clinical grounds in a compatible epidemiological context, with laboratory testing serving mainly to confirm for public health reporting/outbreak-control purposes rather than to establish the diagnosis from a position of genuine clinical uncertainty.

Treatment

There is no specific antiviral treatment for measles — management is entirely supportive. Vitamin A supplementation, genuinely notable as a specific, evidence-based intervention in this viral disease, is recommended for all children with acute measles (particularly in resource-limited settings/regions with underlying vitamin A insufficiency), since it has been shown to reduce measles-associated morbidity and mortality, reflecting vitamin A’s broader role in maintaining epithelial/mucosal integrity and immune function that measles itself disrupts.

Prevention: Vaccination

The live attenuated MMR (measles-mumps-rubella) vaccine — combining all three organisms covered across this and the next two topics into a single formulation — is highly effective, given per the National Immunization Schedule (see Vaccines and Immunoprophylaxis) as a first dose around 9–12 months and a second dose in early childhood. Given measles’ exceptionally high R₀ and correspondingly high herd-immunity threshold described above, any meaningful gap in population vaccine coverage rapidly translates into outbreak risk — a genuinely important, real-world pattern seen repeatedly in vaccine-hesitancy-driven outbreaks in various countries, and a direct, practical illustration of why herd immunity thresholds aren’t just an abstract epidemiological concept but a hard operational requirement specifically for a pathogen this transmissible. Post-exposure prophylaxis with either the vaccine itself (if given within 72 hours of exposure) or measles immunoglobulin (for exposed individuals who cannot safely receive the live vaccine — infants too young, pregnant women, the immunocompromised) can prevent or attenuate disease in a susceptible contact.