Measles spreads through the air when an infected person coughs, sneezes, or even breathes. The virus is extraordinarily contagious, with a basic reproduction number (R0) of 12 to 18, meaning a single infected person can spread it to 12 to 18 others in an unvaccinated population. That makes measles one of the most infectious diseases known to science, far more transmissible than the flu or COVID-19.
How the Virus Travels
Measles lives in the mucus of an infected person’s nose and throat. When that person coughs or sneezes, tiny virus-laden droplets launch into the surrounding air. But unlike many respiratory viruses that drop to the ground quickly, measles can linger in the air for up to two hours after the infected person has left the room. You don’t need to be standing next to someone to catch it. Walking into a room where an infected person coughed an hour earlier is enough.
You can also pick up the virus by touching a contaminated surface and then touching your eyes, nose, or mouth, though airborne transmission is the primary route. The virus initially targets immune cells in the respiratory tract, then spreads through the body via the lymph nodes and bloodstream before eventually reaching the skin, where it produces the characteristic rash.
When an Infected Person Is Contagious
A person with measles is infectious for four days before the rash appears and four days after. This is a critical detail: someone can be spreading the virus before they even know they’re sick. During those early days, the symptoms look like a common cold (fever, runny nose, cough), so an infected person might go to work, school, or the grocery store without realizing they’re exposing everyone around them.
Who Is Most at Risk
Anyone who hasn’t been vaccinated or hasn’t had measles before is susceptible. If you’re unvaccinated and exposed to the virus, your chances of getting infected are extremely high given how efficiently it spreads.
Infants are in a particularly vulnerable window. Babies are born with some protection from their mother’s antibodies, but that shield fades fast. A multi-country study found that by 2.5 to 6 months of age, depending on the population, most infants’ maternal antibody levels drop below the threshold needed for clinical protection. By 6 months, fewer than half of infants in most countries studied still had adequate protection. Since the first MMR vaccine dose isn’t typically given until 12 months of age, babies between roughly 6 and 12 months old have a gap where they’re largely unprotected.
Other high-risk groups include people with weakened immune systems, pregnant women who haven’t been vaccinated, and travelers to regions with active outbreaks.
How Vaccination Changes the Math
One dose of the MMR vaccine is 93% effective against measles. Two doses raise that to 97%. That second dose isn’t a booster in the traditional sense. It catches the small percentage of people whose immune system didn’t fully respond the first time around.
The standard schedule calls for one dose at 12 to 15 months and a second dose at 4 to 6 years. If you’re an adult unsure of your vaccination status, a blood test can check whether you have protective antibody levels. People born before 1957 in the United States are generally considered immune because measles circulated so widely that nearly everyone was infected during childhood.
Why Outbreaks Keep Happening
Because measles is so contagious, it requires extremely high vaccination rates, around 95% of a community, to prevent outbreaks. When coverage dips even slightly, the virus finds pockets of susceptible people and spreads rapidly. In 2023, an estimated 10.3 million people worldwide were infected with measles. Outbreaks are currently occurring in every region of the world, with the largest recent case counts in India, Yemen, Pakistan, and Mexico.
After a temporary decline during the COVID-19 pandemic, when lockdowns disrupted transmission but also disrupted routine vaccinations, measles activity has been climbing again globally and in the United States. Missed or delayed childhood vaccinations during 2020 and 2021 left a cohort of under-vaccinated children now entering school age, fueling new clusters of cases.
What Happens After Exposure
If you’ve been exposed to measles and you’re not immune, symptoms typically begin 7 to 14 days later. The illness unfolds in stages. First comes a high fever, often reaching 104°F or higher, along with cough, runny nose, and red, watery eyes. Small white spots may appear inside the mouth. Two to four days after these initial symptoms, the rash breaks out, usually starting on the face and spreading downward across the body.
Most people recover within two to three weeks, but measles can cause serious complications including pneumonia, brain swelling, and in rare cases, death. Children under 5 and adults over 20 are most likely to experience severe outcomes. The virus also temporarily suppresses the immune system for weeks to months afterward, leaving a person more vulnerable to other infections even after they’ve recovered from measles itself.