Measles spreads through the air when an infected person coughs or sneezes

Measles is caused by a virus that travels in tiny droplets released when someone with measles coughs, sneezes, or speaks. If you breathe in those droplets, or if they land on a surface you touch and then you touch your face, you can become infected. The virus enters through your nose, mouth, or eyes and begins to multiply in your body.

The virus is extremely contagious. If you are not vaccinated and spend time in the same room as someone with measles, your chance of catching it is about 9 in 10. This high transmission rate is why measles spreads so quickly through communities, especially in places where many people are unvaccinated.

You can catch measles from someone even before they develop the typical rash. In fact, people are most contagious during the first four days after the rash appears, though they can spread the virus for several days before symptoms show up.

Key Takeaways

  • Measles spreads through respiratory droplets when an infected person coughs or sneezes, and you can catch it by breathing those droplets in or touching contaminated surfaces.
  • An unvaccinated person exposed to measles has about a 90 percent chance of becoming infected.
  • People with measles are contagious from about four days before the rash appears until four days after it develops.
  • The MMR vaccine prevents measles infection in about 97 percent of people who receive both doses.

How long the virus survives on surfaces and in the air

Measles droplets can float in the air for up to two hours after an infected person coughs or sneezes. This means you can catch measles even if the person who was sick has left the room. The virus can also land on doorknobs, railings, and other surfaces and remain infectious for several hours.

This is why measles can spread in schools, public transportation, hospitals, and other crowded spaces. Even brief contact—sitting near someone on a bus or in a waiting room—carries real risk if you are not vaccinated.

Who is most likely to catch measles

Anyone who has not been vaccinated against measles or who has not had the disease before can catch it. Babies under one year old are at particular risk because they are too young to receive the MMR vaccine, which is normally given starting at 12 months. Pregnant women who are not immune can also become infected, and measles during pregnancy can harm the developing baby.

People with weakened immune systems—including those with HIV, those undergoing cancer treatment, or those taking certain medications—have a harder time fighting off the infection even if they were vaccinated. Some people who were vaccinated decades ago may have lost immunity over time, though this is uncommon.

Unvaccinated children and adults make up the largest group at risk. In communities where vaccination rates drop below a certain threshold, measles can spread rapidly through the unvaccinated population.

What happens in your body after exposure

After you breathe in the measles virus, it takes about 10 to 14 days before you develop symptoms. During this time—called the incubation period—you may feel fine and have no idea you are infected. However, you can still spread the virus to others during this period, which is why measles outbreaks can grow quickly before anyone realizes what is happening.

Once symptoms begin, they typically start with fever, cough, runny nose, and red eyes. A few days later, the characteristic measles rash appears, usually starting on the face and spreading downward. By the time the rash shows up, you have already been contagious for several days.

Why vaccination is the main way to prevent infection

The MMR vaccine (which protects against measles, mumps, and rubella) is the most effective way to prevent measles. One dose provides about 93 percent protection, and two doses provide about 97 percent protection. The vaccine contains a weakened form of the virus that teaches your immune system to recognize and fight measles without causing the disease itself.

Most children receive their first MMR dose at 12 months and their second dose between 4 and 6 years old. Adults born in 1957 or later who have not been vaccinated or who are unsure of their vaccination status can receive the vaccine at any time. A single dose is often enough for adults, though two doses provide stronger protection.

If you are exposed to measles and you are not vaccinated, receiving the MMR vaccine within 72 hours of exposure may prevent infection or reduce how severe the illness becomes. This is called post-exposure vaccination.

What to do if you think you have been exposed

If you know you have been around someone with measles and you are not vaccinated, contact your doctor or local health department right away. They can determine whether you need the vaccine or whether you should be monitored for symptoms. Do not go to a doctor's office or hospital without calling first, because you could spread the virus to others in the waiting room.

If you develop symptoms like fever, cough, and rash after a known exposure, isolate yourself at home and call your doctor before visiting in person. Most people recover from measles on their own, but a doctor needs to confirm the diagnosis and watch for complications, especially in young children, pregnant women, and people with weakened immune systems.

Measles outbreaks and community spread

Measles outbreaks happen when vaccination rates in a community drop too low. When enough people are vaccinated, the virus cannot find new people to infect and transmission stops—a concept called herd immunity. For measles, about 95 percent of a population needs to be immune (either through vaccination or past infection) to prevent outbreaks.

Even small pockets of unvaccinated people can trigger large outbreaks. Travelers who bring measles back from other countries can spark chains of transmission in communities with low vaccination rates. Once an outbreak starts, it spreads quickly because the virus is so contagious.

Public health agencies track measles cases and work to contain outbreaks by identifying exposed people, confirming diagnoses, and encouraging vaccination in affected areas.

Frequently Asked Questions

Can you catch measles twice?

No. Once you have had measles or been vaccinated against it, your body develops lifelong immunity. You cannot catch measles a second time. This is why vaccination provides permanent protection.

Can you catch measles from the MMR vaccine?

No. The MMR vaccine contains a weakened virus that cannot cause measles. You cannot catch measles from the vaccine itself. Some people develop a mild rash a week or two after vaccination, but this is not measles and is not contagious.

How long after exposure do symptoms appear?

Symptoms usually appear 10 to 14 days after exposure, though the range can be 7 to 21 days. During this time, you may spread the virus to others without knowing you are infected. This is why people exposed to measles should watch for symptoms and contact their doctor if fever or rash develops.

Can you catch measles from someone who is vaccinated?

Very rarely. Vaccinated people have strong protection against measles infection. In the uncommon cases where a vaccinated person does get measles, they typically have milder symptoms and are less contagious than unvaccinated people. The vaccine significantly reduces the chance of both catching and spreading measles.

What should I do if I am pregnant and exposed to measles?

Contact your doctor immediately. Measles during pregnancy can cause serious complications for both you and your baby, including miscarriage and birth defects. Your doctor can assess your immunity status and discuss your options. If you are not immune, your doctor will monitor you closely for symptoms.