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

The measles virus travels in tiny droplets that an infected person releases into the air. When you breathe in those droplets, the virus can enter your respiratory tract and begin to multiply. This is why measles is one of the most contagious infections known — a single infected person will transmit the virus to about 9 out of 10 unvaccinated people who are near them.

You do not need to touch an infected person or share food or drinks to catch measles. Simply being in the same room for 15 minutes or more puts you at risk if the infected person has coughed or sneezed recently. The virus can linger in the air or on surfaces for up to two hours after an infected person leaves.

The virus enters through your nose or mouth and infects cells in your respiratory tract. From there, it spreads to your lymph nodes and eventually throughout your body, which is why measles affects multiple organ systems and causes the characteristic rash.

Key Takeaways

  • Measles spreads through respiratory droplets when an infected person coughs, sneezes, or breathes, not through touch or shared food.
  • The virus can remain in the air or on surfaces for up to two hours, so you can catch measles even if the infected person has left the room.
  • An unvaccinated person exposed to measles has about a 90 percent chance of becoming infected.
  • You are contagious from four days before your rash appears until four days after it appears, meaning you can spread measles without knowing you have it.
  • The MMR vaccine prevents measles infection in about 97 percent of people who receive both doses.

When you are most contagious

You can spread measles to others starting four days before your rash appears. This is the most dangerous period because you may not yet know you have measles — you might think you have a cold or flu. During this time, you have a fever, cough, and runny nose, and the virus is actively multiplying in your respiratory tract.

You remain contagious for four days after your rash first appears. After that point, your immune system has begun to control the infection and you shed fewer viral particles into the air. Most people are no longer contagious by the time the rash has covered their entire body, though this varies slightly from person to person.

This eight-day window of contagiousness — four days before symptoms and four days after the rash starts — is why measles spreads so quickly through communities. People go to work, school, or public places while contagious but before they realize they are sick.

Who is at highest risk of catching measles

Unvaccinated people face the highest risk. If you have not received the MMR vaccine or have received only one dose instead of two, your risk of infection after exposure is very high. Infants under 12 months old cannot yet receive the vaccine, so they depend on immunity from their mothers or on protection from vaccinated people around them.

People with weakened immune systems — including those with HIV, those undergoing chemotherapy, or those taking immunosuppressive medications — have difficulty fighting off the infection even if vaccinated. Pregnant people who are not immune should avoid exposure, since measles infection during pregnancy increases the risk of miscarriage and premature birth.

People born before 1957 are generally considered immune because measles was so common that most people caught it naturally. However, some people born in that era were vaccinated with an inactivated vaccine that provided incomplete protection, so immunity status can vary.

How the virus survives outside the body

Measles virus particles can remain viable on surfaces or in the air for up to two hours. This means you could catch measles by touching a contaminated surface and then touching your face, though this is less common than direct inhalation of respiratory droplets. The virus is fragile and does not survive long in the environment compared to some other viruses.

Standard cleaning with soap and water or disinfectant breaks down the virus and eliminates it from surfaces. Healthcare settings use this fact to control measles spread — thorough cleaning of patient rooms and equipment is part of standard infection control.

Measles in healthcare settings and outbreaks

Healthcare workers and patients in hospitals face particular risk because measles spreads rapidly in enclosed spaces with recirculated air. A single infected patient can expose dozens of staff members and other patients. Hospitals manage this by isolating measles patients in negative-pressure rooms, where air is drawn out of the room rather than circulating to other areas.

In community outbreaks, measles spreads fastest in places where unvaccinated people gather — schools with low vaccination rates, childcare centers, or religious communities with vaccine hesitancy. Once measles appears in such a setting, it can infect a large percentage of the unvaccinated population within weeks.

What happens after you are exposed

If you are exposed to measles, symptoms typically appear 10 to 14 days later, though the range can be 7 to 21 days. During this incubation period, you have no symptoms but the virus is multiplying in your body. You are not yet contagious during the first few days of incubation, but by day 4 or 5 after exposure, you may begin shedding virus into the air even though you feel fine.

The first signs are fever, cough, runny nose, and red eyes — symptoms that look like a common cold or flu. A few days later, small white spots appear inside your mouth (called Koplik spots), followed by the characteristic measles rash that starts on your face and spreads downward.

Frequently Asked Questions

Can you catch measles twice?

No. Once you have had measles or been vaccinated against it, your immune system produces antibodies that protect you for life. Reinfection is extremely rare and occurs only in people with severe immune system problems. Natural infection provides lifelong immunity.

Can you catch measles from someone who was vaccinated?

No. The MMR vaccine contains a weakened form of the virus that cannot cause measles infection. Vaccinated people do not shed measles virus. In rare cases, people vaccinated with the live vaccine shed a small amount of vaccine virus, but this does not cause measles disease in others.

How long after exposure should you watch for symptoms?

Watch for symptoms for up to three weeks after exposure, though most people develop them between 10 and 14 days. Early symptoms — fever, cough, runny nose, and red eyes — can look like other illnesses. If you develop these symptoms after known measles exposure, contact your doctor before visiting in person so they can take precautions.

What should you do if you think you have been exposed?

Contact your doctor or local health department to report the exposure. They can advise whether you need post-exposure prophylaxis (a treatment given within 72 hours of exposure that can prevent infection) and whether you should be tested. Do not visit a clinic or hospital without calling first, as this could expose others.

Can you catch measles from contaminated food or water?

No. Measles virus infects the respiratory tract, not the digestive system. The virus is destroyed by stomach acid and digestive enzymes, so it cannot cause infection if swallowed. Measles spreads only through respiratory droplets or direct contact with respiratory secretions.