Measles Virus Explained — Why It's Coming Back in 2026

Measles is among the most contagious viruses that infect people: according to the World Health Organization, one infected person can generate up to 18 secondary infections, and the virus stays infectious in the air for up to two hours after the person has left the room. A safe, cheap vaccine exists, and it brought global measles deaths down by about 88 percent between 2000 and 2024. Yet in 2026 the United States has recorded more cases than in any recent year, Europe has had its worst outbreak in decades, and the Americas lost their regional elimination status. This article explains how the virus works, what it does inside the body, and why vaccination coverage is the whole story.

Educational content, not medical advice. This article describes general, well-established science and public health data. Questions about your own or your child's vaccination status belong with a clinician.

What measles actually is

Measles is caused by the measles virus, an enveloped virus with a genome made of a single strand of RNA, in the genus Morbillivirus within the family Paramyxoviridae. Its closest relatives infect animals, including the viruses that cause canine distemper and the cattle disease rinderpest. Humans are the only known host of measles. There is no animal reservoir where it can hide between outbreaks, which is one reason eradication is considered biologically feasible.

The particle is roughly spherical and a little over 100 nanometers across, wrapped in a membrane that carries two proteins that do most of the work. The hemagglutinin (H) protein binds receptors on the host cell. The fusion (F) protein then pulls the viral membrane into the cell membrane so the genome can enter. Inside, the virus replicates in the cytoplasm rather than the nucleus, and it has no mechanism for integrating into host DNA.

The virus is genetically stable in the ways that matter for vaccination. There is effectively one measles type in terms of what antibodies recognize, so immunity from either infection or vaccination protects against all circulating strains. That is why a vaccine whose basic design dates to the 1960s still works. The same stability is the reason measles can be tracked precisely: public health laboratories sequence the virus, group it by genotype and lineage, and use those groupings to decide whether two outbreaks are connected, as discussed in the elimination section below.

For readers new to the underlying biology, our guide to how viruses work explains how enveloped RNA viruses enter cells and take over their machinery.

Why measles spreads so easily

Contagiousness is measured by the basic reproduction number, R0 (pronounced R-naught): the average number of people one case infects in a population with no immunity. Measles is usually cited at 12 to 18, a range reviewed in a 2017 systematic review in The Lancet Infectious Diseases by Guerra and colleagues. The WHO uses the upper end of that range when it says one case can generate up to 18 secondary infections.

Several features push that number up.

• Airborne spread. Measles travels as fine respiratory droplets that stay suspended. WHO and the CDC both say the virus can remain infectious in the air for up to two hours after an infected person leaves a space. A person can catch measles by walking into a room that is now empty.
• Infectiousness before diagnosis. A person can transmit measles from four days before the rash appears to four days after. By the time the distinctive rash identifies the illness, the person has typically been infectious, and moving through public spaces, for days.
• High attack rate. The CDC says up to nine in ten susceptible people who have close contact with a measles patient will develop measles.
• Very high viral output. Infected cells produce large numbers of virus, so each cough carries a heavy dose.

The arithmetic of R0 explains why the vaccination target is so high. If each case infects 12 to 18 others when nobody is immune, the chain of transmission only stops when most contacts are already protected. WHO says at least 95 percent coverage with two doses is needed to stop transmission and protect communities, and some modeling work argues that for a virus this transmissible even 95 percent leaves little margin. The threshold works like a levee: a river can stay within its banks at 95 percent of the flood, and one stretch of low wall is enough for the water to find it. This is why measles tends to appear first in specific schools, neighborhoods, and religious or social communities where coverage is low, even when national averages look reasonable.

How the virus moves through the body

Measles infection is a tour of the immune system, not just the airway. The sequence, drawn from the standard picture of measles pathogenesis, runs as follows.

1. Entry. The virus is inhaled or lands on the surface of the eye. Its first targets are immune cells in the airways, such as alveolar macrophages and dendritic cells, which carry a surface protein called CD150, also known as SLAM (signaling lymphocytic activation molecule).
2. Carriage to lymph nodes. The infected immune cells travel to nearby lymph nodes, where the virus replicates heavily in lymphocytes, the white blood cells that include T cells and B cells.
3. Spread through the body. A first wave of virus in the blood seeds lymphoid tissue across the body, including the spleen, tonsils, and thymus.
4. Return to the surface. A second wave carries infected cells back to the airway lining, the skin, and other surfaces. Here the virus uses a second receptor, nectin-4, which sits on epithelial cells. Two papers published in 2011, one in Nature by Mühlebach and colleagues and one in PLoS Pathogens by Noyce and colleagues, identified nectin-4 as the epithelial receptor, which explained how a virus that begins in immune cells reaches the airway lining and can be coughed out again.
5. Symptoms. The rash and the respiratory symptoms appear when the immune system recognizes and attacks the infected cells, which is why the rash does not appear on day one.

The CDC lays out the visible timeline. Fever, cough, runny nose, and red watery eyes begin 7 to 14 days after exposure, and the fever can exceed 104°F. Tiny white spots inside the mouth, called Koplik spots, may appear two to three days after symptoms begin. The rash starts three to five days after symptoms begin as flat red spots at the hairline, spreads down the neck, trunk, arms, legs, and feet, and may join together while the fever spikes again.

The rash itself is a useful clue to how the disease works. It is not caused by the virus bursting cells in the skin. It reflects an immune response, which means that people with seriously weakened immune systems can have severe measles without a typical rash.

Immune amnesia: the part that outlasts the infection

The feature of measles that has changed how scientists think about it is what happens afterward. Measles infects memory T cells and B cells, the cells that preserve the immune system's record of every pathogen it has met. Losing some of them is like losing pages from a medical file: the body is still alive and still has an immune system, but part of the record of what it knows how to fight is gone.

The best-known direct evidence comes from a 2019 study in Science by Mina and colleagues. They used a technique called VirScan to measure antibodies against thousands of viral and bacterial targets in 77 unvaccinated children in the Netherlands, before and after a 2013 measles outbreak. Measles eliminated between 11 and 73 percent of the children's antibodies against other pathogens, with wide variation between individuals. A companion paper in Science Immunology by Petrova and colleagues found that the pool of B cells recovers incompletely. The effect is not permanent in the sense that the immune system continues to learn, but it can leave a child exposed to infections they were previously protected against.

The CDC now says directly that measles can weaken the immune system for months or up to a few years, an effect it calls immune amnesia. This helps explain a pattern that long puzzled epidemiologists: in countries where measles vaccination expanded, childhood deaths fell by more than measles alone accounted for. Analyses of historical records by Mina and colleagues, published in Science in 2015, pointed to the same conclusion. Fewer measles infections meant fewer cases of immune memory loss and fewer deaths from other infections. Exact estimates of the share of non-measles deaths prevented depend heavily on setting and method, so they are not quoted here.

Two caveats are worth stating. The 2019 study measured antibodies in a small group, 77 children, and antibodies are only one part of immunity. And the research concerns infection with wild measles virus, not the weakened virus in the vaccine. The practical conclusion is that measles prevention protects children against more than measles. Our overview of how the immune system works covers the memory cells involved.

Complications and who is at risk

Most people recover, but measles is not a mild disease on average. The CDC gives approximate figures for what can go wrong.

Complication	Approximate frequency (CDC)
Ear infection	About 1 in 10 children
Diarrhea	Fewer than 1 in 10 people
Pneumonia	Up to 1 in 20 children
Encephalitis (brain inflammation)	About 1 in 1,000 children
Death	Roughly 1 to 3 per 1,000 infected children, from respiratory or neurologic complications
Hospitalization	About 1 in 5 unvaccinated people in the U.S. who get measles

Source: CDC, measles signs and symptoms.

The highest-risk groups are children under 5, adults over 20, pregnant women, and people with weakened immune systems. In pregnancy, measles can lead to premature birth or a low-birth-weight baby. Pneumonia is a leading cause of measles deaths. It can result from the virus itself or from bacteria that take advantage while the immune system is suppressed.

Subacute sclerosing panencephalitis (SSPE) deserves its own mention because it shows how far a measles infection can reach in time. It is a rare, fatal brain disease caused by a persistent form of the virus, and the CDC says seizures generally begin 7 to 10 years after the original infection. Risk estimates vary between studies and are higher for infants infected before their first birthday, which is one reason the protection of babies too young to be vaccinated depends on everyone around them.

Death rates are far higher where nutrition and medical care are poor. Vitamin A deficiency worsens measles, which is why WHO recommends two doses of vitamin A for children under 5 with suspected measles. There is no specific antiviral treatment, so care focuses on relieving symptoms, maintaining hydration, and treating complications.

The MMR vaccine

The measles, mumps, and rubella vaccine (MMR) is a live attenuated vaccine, meaning it contains weakened viruses that replicate enough to train the immune system but not enough to cause measles in healthy people. The history is documented in the CDC's Pink Book: John Enders and Thomas Peebles isolated the measles virus in 1954, the first live attenuated measles vaccine was licensed in the United States in 1963, and the combined MMR vaccine was licensed in 1971. An earlier killed vaccine used from 1963 to 1967 was not effective, which is why people vaccinated in that window with unknown vaccine type should be revaccinated, per the CDC's vaccine page.

On effectiveness, the CDC reports that one dose is about 93 percent effective against measles and two doses about 97 percent. Antibodies develop in about 95 percent of children vaccinated at 12 months, and more than 99 percent of those who receive two doses show evidence of immunity.

	One dose	Two doses
Measles effectiveness	About 93%	About 97%
Mumps effectiveness	About 72%	About 86%
Rubella effectiveness	About 97% (likely lifelong)	Not applicable

Source: CDC MMR vaccine information.

Schedules differ by country. In the United States, children receive two doses, at 12 to 15 months and at 4 to 6 years. WHO recommends a first dose at 9 months in high-burden countries and at 12 to 15 months elsewhere, with a second dose usually at 15 to 18 months. A second dose is not a booster for waning protection. It mainly catches the small share of children who did not respond to the first, and the CDC notes that almost everyone who does not respond to the first dose will respond to the second.

Adults without evidence of immunity need at least one dose, and some groups need two, including health care personnel, college students, and international travelers. People born before 1957 are generally presumed immune because measles circulated so widely before the vaccine. MMR given within 72 hours of an exposure may provide some protection or milder illness. These are general public health recommendations rather than advice for any individual, and a clinician can check records and circumstances. The mechanics of how vaccines build protective memory are described in our explainers on how vaccines train memory B cells and how vaccines work, and the broader history is in how vaccines were invented.

Safety and the autism claim

Common side effects are mild: soreness, fever, a mild rash, or temporary joint pain. Serious reactions are rare, and the CDC identifies a seizure caused by high fever as the main concern. The agency states plainly: "There is no link between the MMR vaccine and autism."

The claim originated in a 1998 paper by Andrew Wakefield and colleagues in The Lancet. After a U.K. General Medical Council panel ruled on January 28, 2010 that Wakefield had acted unethically, the journal formally retracted the paper in early February 2010 and said that claims in it, including that the children had been consecutively referred and that the investigations had ethics committee approval, were false. A large body of subsequent research has not found an association. Our article on how we know vaccines are safe walks through the monitoring systems that look for rare harms.

What the numbers show: 2000 to 2026

The global picture has two sides. Vaccination has been a large success: WHO estimates measles deaths fell from about 780,000 in 2000 to about 95,000 in 2024, a drop of roughly 88 percent, with nearly 59 million deaths averted by vaccination over that period. Before the vaccine existed, WHO says, epidemics occurred every two to three years and caused about 2.6 million deaths per year. In the United States before vaccination, the CDC says about 500,000 cases and 500 deaths were reported each year, though the true number of cases was likely 3 to 4 million.

The other side is that coverage has slipped. WHO reports global first-dose coverage of 84 percent in 2025, below the 2019 level of 86 percent, and second-dose coverage of 77 percent. An estimated 29 million infants were under-protected in 2025. The vaccine costs less than one U.S. dollar per child, so the barrier is rarely price alone.

Regional numbers show what that gap produces.

• Europe. WHO and UNICEF reported 127,350 measles cases in the European Region in 2024, about double the 2023 figure and the highest since 1997. More than half of reported cases needed hospital care, children under five made up more than 40 percent, and 38 deaths had been reported as of March 6, 2025. Romania reported 30,692 cases and Kazakhstan 28,147. In 2023, fewer than 80 percent of eligible children in Bosnia and Herzegovina, Montenegro, North Macedonia, and Romania had received a first dose, according to the WHO Europe release.
• The Americas. After the Pan American Health Organization (PAHO) found that endemic transmission had been re-established in Canada, the region lost its status as free of endemic measles in November 2025, per PAHO.
• The United States. The CDC counts are in the table below.

Year	U.S. confirmed cases	Outbreaks (3 or more related cases)
2019	1,274	Not stated
2024	285	16
2025	2,289	48
2026 (through Oct. 15)	4,410	42

Source: CDC measles data, page updated October 16, 2026. 2026 figures are provisional and change weekly.

The CDC says 96 percent of the 2026 cases so far were outbreak-associated, and the most recent hospitalization breakdown available for 2024 shows 40 percent of cases (114 of 285) were hospitalized, including 52 percent of children under 5. Those figures change as reports come in, so the live CDC page is the right place for current counts.

Elimination status and what "lost" means

A country has eliminated measles when it has gone without continuous domestic spread for a defined period. PAHO defines endemic transmission as "uninterrupted circulation of the same virus genotype and lineage for 12 months or more in a specific geographic area." The question for the United States is whether the large outbreak that began in West Texas on January 20, 2025 and later outbreaks form one unbroken chain. PAHO says it will review the status of the United States and Mexico in November 2026, partly to allow time for complete genome sequencing, so the formal answer is pending as of this writing.

This is why the technical details of the virus matter for policy. Because measles lineages can be fingerprinted, the question is not whether many cases occurred, which is already known, but whether the same lineage kept circulating for a full year. Losing elimination status does not change what happens to a person who gets measles. It signals that the national defenses are no longer reliably stopping chains of transmission.

Why cases are rising

The causes sit in several layers, and they differ between regions.

• Interrupted routine immunization. The COVID-19 pandemic disrupted childhood vaccination programs, and WHO's 2025 coverage figures remain below pre-pandemic levels.
• Local pockets of low coverage. National averages hide communities where uptake is far below 95 percent. A single large outbreak can start in such a pocket and then reach others, which is what outbreak investigations repeatedly find.
• Access and conflict. In countries affected by conflict, displacement, or weak health systems, children miss routine doses simply because services are not reachable.
• Hesitancy and misinformation. Concerns about vaccine safety, amplified online, reduce uptake in some communities, even where the evidence is clear.
• Travel. Measles carried by travelers can ignite outbreaks wherever susceptible people cluster. The CDC counted 26 cases among international visitors in 2025 and 18 so far in 2026.

No single factor explains the surge, and policy responses that treat it as only an information problem or only an access problem tend to miss half the picture.

Common misconceptions

"Measles is a mild childhood illness." Many people recover, but the CDC lists pneumonia in up to 1 in 20 children and 1 to 3 deaths per 1,000 infected children, with higher risks for infants, pregnant women, and immunocompromised people.

"If you had measles as a child you are not protected." The opposite is true. Natural infection is believed to confer lasting immunity, but the infection itself can erase part of the immune memory against other diseases.

"Vaccinated people spread measles just as much." Two doses are about 97 percent effective. A small number of vaccinated people can still be infected, but outbreaks overwhelmingly involve people who were unvaccinated or whose status is unknown, which was 89 percent of cases in 2024 per the CDC.

"Vitamin A treats measles." Vitamin A is recommended by WHO for children with suspected measles to reduce severity, especially where deficiency is common, but it does not cure the infection and is not a substitute for vaccination.

"Measles can't come back in a rich country." The 2024 to 2026 data show it can. The virus does not depend on poverty. It depends on susceptible people sitting close together.

What this means

Measles is a test of public health infrastructure because of one number: 95 percent. The biology is not a mystery. The virus is stable, humans are its only host, the vaccine works and has barely changed in decades, and the damage the infection does to immune memory is now documented. What remains is arithmetic and logistics. When the share of children protected falls below the level needed to stop transmission, measles is the first disease to show it, and it does so quickly. The recent rise in Europe and the United States, and the loss of regional elimination status in the Americas, are signals about coverage rather than about any change in the virus. The same vaccine that works in 2026 worked in the 1960s, and the places that restore high two-dose coverage see cases fall again.

Frequently asked questions

Q: I had measles as a child. Am I still immune? A: Almost certainly yes. Natural infection is believed to produce long-lasting immunity, and the CDC says vaccine-induced immunity is also long-term in most people. Anyone unsure of their history can ask a clinician about checking records.

Q: I was born before 1957. Do I need MMR? A: The CDC says people born before 1957 are generally presumed immune because measles circulated widely before the vaccine. Health care personnel born before 1957 without laboratory evidence of immunity should consider two doses.

Q: Can adults get measles? A: Yes. The CDC lists adults over 20 among the highest-risk groups for complications. Adults without evidence of immunity need at least one dose, and some groups, such as health care personnel and international travelers, need two.

Q: How contagious is measles compared with other infections? A: Its R0 is usually cited at 12 to 18, and WHO says one case can generate up to 18 secondary infections. The CDC adds that up to nine in ten susceptible close contacts will become infected.

Q: What does the measles rash look like and when does it appear? A: It begins as flat red spots at the hairline three to five days after the first symptoms, then spreads down the neck, trunk, arms, legs, and feet, and may merge. Fever can spike above 104°F when the rash appears, per the CDC.

Q: Can vaccinated people get measles? A: Rarely. Two doses are about 97 percent effective, so a small number of vaccinated people remain susceptible. Most cases in recent U.S. outbreaks have been among unvaccinated people or people whose status is unknown.

Q: Is MMR linked to autism? A: No. The CDC states there is no link between the MMR vaccine and autism. The 1998 paper that proposed one was retracted by The Lancet in 2010.

Q: What is SSPE? A: Subacute sclerosing panencephalitis is a rare, fatal brain disease caused by persistent measles virus. The CDC says seizures generally develop 7 to 10 years after infection, and there is no cure.

Q: Why hasn't measles been eradicated like smallpox? A: The barriers are coverage and logistics, not biology. Humans are the only host and the vaccine works well, but with 84 percent first-dose coverage worldwide in 2025, transmission continues in under-vaccinated populations.

Sources

• WHO. Measles fact sheet, July 15, 2026.
• CDC. Measles cases and outbreaks data, updated October 16, 2026.
• CDC. Measles clinical overview, signs and symptoms, and MMR vaccine information.
• CDC. Pink Book, chapter 13: measles.
• PAHO. Update on the review of measles elimination status, March 2, 2026, and measles elimination status in the United States and Mexico, January 16, 2026.
• WHO Europe and UNICEF. European Region reports highest number of measles cases in more than 25 years, March 13, 2025.
• Mina MJ, et al. Measles virus infection diminishes preexisting antibodies that offer protection from other pathogens. Science 2019;366:599-606.
• Mina MJ, et al. Long-term measles-induced immunomodulation increases overall childhood infectious disease mortality. Science 2015;348:694-699.
• Petrova VN, et al. Incomplete genetic reconstitution of B cell pools contributes to prolonged immunosuppression after measles. Science Immunology 2019.
• Guerra FM, et al. The basic reproduction number (R0) of measles: a systematic review. Lancet Infect Dis 2017;17:e420-e428.
• Noyce RS, et al. Tumor cell marker PVRL4 (nectin 4) is an epithelial cell receptor for measles virus. PLoS Pathog 2011;7:e1002240.

For related virus coverage, see our explainers on HIV, H5N1 bird flu, and HPV. The story of the first vaccine is told in our article on Jenner and the smallpox vaccination.

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