In Pennsylvania, there were roughly 4 times as many measles cases in 2026 as in the previous 3 decades—combined.

Measles cases in the US reached a 35-year high in 2026 as vaccination rates dropped.

A rare measles complication is that the virus can linger in the body for years after apparent recovery and continue to replicate—leading to a progressive and fatal breakdown of the central nervous system that can occur decades after the initial infection.

Measles attacks the body's immune system, leaving it vulnerable to secondary complications.

Measles is one of the most contagious preventable diseases—it was eliminated in the US in 2000, but it is now making a resurgence due to declining vaccination rates.

Discover the latest information on US measles cases from official outbreak tracking figures.

Measles infection, which was nearly universal during childhood in prevaccine era, was described by the Persian physician Rhazes in the 10th century as "more to be dreaded than smallpox."

A look at the 2025 measles outbreak in West Texas and surrounding areas

A measles outbreak in the summer of 2025 ultimately led to more than 900 cases and the deaths of two young children. The highly contagious virus requires vaccination rates near 95% to achieve herd immunity—the Texas outbreak spread through areas of significant mistrust of vaccines, including the area’s Mennonite community.

Measles was declared eliminated in the US in 2000, meaning the country had been free of continuous disease transmission for 12 months or more.

The measles vaccine has saved an estimated 94 million lives since its introduction.

Measles symptoms range from fever to pneumonia and encephalitis

While less common, most measles-related deaths come from secondary conditions such as pneumonia (one to six cases per 100 patients) and encephalitis (one case per 1,000 patients). Blindness, which is rare, may arise from corneal ulceration.

Measles infection can cause long-term immune system “amnesia”

The measles virus attacks the body's immune system, including preying on T and B cells in the lymphatic system. These cells hold memories of past infections, which can be degraded as the virus destroys them. This leaves patients susceptible to other infections for a year or longer.

A 1954 outbreak at a Boston boarding school allowed scientists to isolate the measles virus and formed the basis for the first measles vaccine.

Secondary complications from measles in children range across the whole body.

Measles evolved from an ancient cattle disease, branching off as recently as 3,000 years ago.

Explore a technical look at the measles virus and its toll on humanity through the ages.

What is the measles, mumps, and rubella vaccine?

Federal guidance currently recommends a two-shot schedule of the MMR vaccine for children in most cases, once at 12 to 15 months and once at 4 to 6 years. (A September 2025 update only called for the elimination of a combined vaccine that included protection against varicella, the MMRV shot.)

Vitamin A does not cure measles, but it helps reduce the symptoms

Also known as retinol, vitamin A plays a significant role in supporting the body’s immune system. The virus attacks the immune system, and studies have shown a correlation between vitamin deficiency and severity of measles symptoms. Supplemental vitamin A doses are now a part of recommended treatment for pediatric patients.

The R0 value is how epidemiologists quantify disease contagiousness

The basic reproduction number indicates the average number of people a person with a disease will infect. Measles is the most contagious disease, with an R0 of 12-18, though vaccines have significantly reduced the number of susceptible individuals.

CDC does not require docs to report hand, foot, and mouth cases which makes tracking difficult

Mid-Atlantic states saw a surge of hand, foot, and mouth disease in 2025 for as-yet unknown reasons. Virginia, Maryland, and New Jersey, reported elevated outbreak numbers in 2025 as compared to other recent years, but since the US Centers for Disease Control and Prevention does not require mandatory reporting of positive HFMD cases—unlike strictly notifiable diseases like measles—it's hard to truly know the extent of the outbreaks and how significant the caseload is as compared to prior years across the region or country.

Understanding R0, a measure of how contagious a pathogen is

The R0 value is a basic measure of how many people an infected person will transmit a pathogen to, on average. If R0 is less than 1, transmission dies out; if greater than 1, an outbreak is likely. Measles has one of the highest known R0 values at almost 18, meaning that after four transmission steps, more than 100,000 people could be infected.

Measles reached global transmission during the 15th- and 16th-century Columbian exchange

Devastating diseases like smallpox, influenza, measles, and other viruses were largely introduced into the Western Hemisphere beginning with the 1492 arrival of Christopher Columbus and the ensuing age of exploration.

Environmental conditions, shifting immunity, and viral strain could help explain 2025 spike

The surge in HFMD in the summer and early fall of 2025 in the mid-Atlantic could be the result of a confluence of factors, including a more virulent strain of disease-causing virus, good environmental conditions for the virus to spread—hot, humd—and a potential immunity gap among kids due to milder HFMD outbreaks the previous year.

See MS prevalence by region in the United States

This map underscores that people in the northeastern United States have more cases of the disease as compared to the south, west, and midwest. On an individual level, these numbers may still differ based on genetic makeup and other factors irrespective of where people live.

See the total number of Lyme cases per state in 2023

New York, Pennsylvania, and Massachusetts had the highest total number of Lyme disease cases in 2023. New York had more than 22,000 cases that year. Maine had one of the highest Lyme incidence rates of disease that year—213 per 100,000 people—but only 2,942 cases overall.

See colorized mpox virus particles grown in culture at Fort Detrick, Maryland, where high-biosecurity pathogens are studied.

A heatmap showing the drop in infectious diseases post-vaccine deployment

The number of infected people, measured over 70-some years and across all 50 states and the District of Columbia, declined significantly after vaccines were introduced.

Diphtheria was once a leading childhood killer before antibiotics and vaccines

In 18th- and 19th-century outbreaks, some New England families lost as many as eight children to diphtheria, and fatality rates in hospitals often topped 60%. The invention of antitoxins, penicillin, and vaccines dramatically reduced deaths.

Up to 225 cases of leprosy in the US are diagnosed each year

Most cases in the US are imported, meaning someone was infected in a country where it was more common. Since symptoms can sometimes take decades to appear, tracing the origin of the disease can be hard.

Amish kids have far fewer allergies than the general population

Only about 7% of Amish children in a northern Indiana community have tested positive for common allergens in routine skin prick tests, whereas half of the general US population tests positive for them. Asthma is also extremely uncommon among the Amish, even when compared with other farming communities. Researchers are interested in exploring if approaches like exposing kids across the general population to Amish farm dust might stave off some allergies.

Improved 19th-century sanitation meant kids encountered polio later, with worse results

Almost all children likely used to contract poliovirus, but scientists suggest that when infants encountered it very early in life—due to poorer conditions and hygiene—they still had some maternal protection from pregnancy or through breastmilk, so the symptoms were more flu-like.

Oral polio vaccines cause approximately 250 to 500 polio cases per year globally

Live, weakened polioviruses in the oral vaccine can very rarely mutate into a disease-causing form and lead to vaccine-associated paralytic poliomyelitis. This oral vaccine is not available in the United States.

10 locally acquired malaria cases were reported in the US in 2023—a surprise

The cases, reported in Florida, Texas, Maryland, and Arkansas, broke a 20-year streak since there had been no reported locally acquired mosquito-transmitted malaria in the country since 2003, when there was a case in Florida.

Eczema appears to heighten the risk of developing shingles.

See the polio cases that occur worldwide each year by region

Worldwide, the number of cases of polio reported each year from 1980 to 2024 has declined. This graphic depicts regional trends and declines by year.

Most polio cases today are vaccine-derived rather than naturally occuring

Weakened, live virus vaccines—which aren't offered in the US—carry a risk of virus being shed in the stool. Rarely, the shed virus mutates and leads to vaccine-derived polio infections. Most recent polio cases are genetically linked to those vaccines. As shown in this graphic, there are typically hundreds of vaccine-derived polio cases annually.

Approximately 70% of all polio infections in children are asymptomatic

Most polio infections are asymptomatic or appear with only flu-like symptoms. Infected individuals can still shed the virus in their stool and mucus for days or weeks

Erythema migrans rash is common in Lyme—but it often doesn't look like a bull's-eye

The US Centers for Disease Control and Prevention states that this rash occurs in roughly 70% or more of Lyme disease patients, and typically occurs at the site of a tick bite. The rash may expand slowly over several days and feel warm to the touch, but it's rarely itchy or painful. As it grows, areas of it may clear—creating a target or "bull's-eye" appearance.

Attenuated vaccines introduce a weakened version of the virus into the body

Some vaccines, like those for measles, rely on weakened versions of a virus. These weakened pathogens are introduced into the body, triggering an immune response and helping the immune system learn to protect itself against stronger versions of the virus. (This also explains why some vaccines make you feel under the weather.)

Nonwhite Lyme patients tend to be diagnosed later and have more severe disease

Peer-reviewed research published in 2025 from Pfizer—a pharmaceutical company working on a candidate vaccine for Lyme disease—examined Lyme data from more than 150,000 Medicare and Medicaid beneficiaries and found that Black patients, as compared to white beneficiaries, were more likely to have disease that had spread further throughout the body at the time of care. The difference, the scientists wrote, may be partly due to difficulties recognizing the characteristic rash on darker-colored skin.

See when the last case of wild poliovirus was found in each country

This chart shows when wild polio cases were last reported in each country worldwide. Beyond wild poliovirus, rare outbreaks can occur related to mutated strains of oral polio vaccine.

Adults usually have some immunity to HFM Disease—but can still spread it

In 2025, certain eastern states including Virginia and West Virginia reported a sizable uptick in their HFMD cases, particularly in their schools and daycare centers, though the reason for the larger outbreaks remains unknown. When adults or teens get the disease, their symptoms are usually mild or even asymptomatic. Still, they may pass the disease between people, too. (Some users may encounter a paywall.)

Before vaccines, there was 'variolation': what was it?

Before the technology behind vaccinations was discovered, some keen observers of medicine around the globe noticed how previous infections could impact future ones. Learn about the techniques early physicians used before they had vaccines.

The shingles vaccine may slow overall biological aging in older adults.

A malaria case in Maryland stumped doctors in 2023—leading to the wrong treatment and underscoring the challenge of physicians being mindful of tropical diseases in our interconnected world.

See Virginia's spike in HFMD cases in 2025

The state's own tracking showed a significant jump in HFMD cases in July 2025 as compared to the prior few years, and that more people were visiting the emergency room and urgent care centers for the disease, but epidemiologists could not explain why the numbers were so high.

Leprosy symptoms may appear 20 years after infection, so contact tracing isn't possible.

About 95% of people are naturally immune to leprosy.

Herd immunity works by slowing down viral transmission until a disease dies out.

See the incidence of Lyme disease across all 50 states in 2023

The US states with the highest rate of Lyme disease—meaning cases per 100,000 people—were Rhode Island, Vermont, and Maine, in 2023. Rhode Island had 260 cases per 100,000 people, based on cases reported to the US Centers for Disease Control and Prevention. (Some users may encounter a paywall.)

HFMD was first reported in 1957 in New Zealand

The first clinical report involved eight kids with the characteristic sores in New Zealand, and then in the years that followed there were dozens more cases there and also reports of outbreaks in England and Scotland, among other settings, including the US.