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

Measles is transmitted when a person with the infection releases tiny droplets into the air by coughing or sneezing. These droplets can travel several feet and land on the nose, mouth, or eyes of another person, or be inhaled directly into the lungs. A single infected person can spread measles to 12 to 18 other people in an unvaccinated group — it is one of the most contagious diseases known.

The virus can also survive on surfaces for up to two hours. If you touch a contaminated surface and then touch your face, you can become infected. However, the primary route of transmission is breathing in the virus directly from an infected person's cough or sneeze.

People are most contagious during the first four days after the rash appears, though they can spread measles from four days before the rash starts until four days after it appears. This means someone can transmit the virus before they even know they have measles.

Key Takeaways

  • Measles spreads through respiratory droplets when an infected person coughs or sneezes, traveling several feet through the air.
  • A person with measles is contagious from four days before the rash appears until four days after it appears.
  • The virus can survive on surfaces for up to two hours, though direct inhalation of droplets is the main way people catch measles.
  • Unvaccinated people in close contact with someone who has measles have a very high chance of becoming infected.

Why measles spreads so easily in groups

Measles is airborne, meaning it does not require close physical contact to spread. In schools, workplaces, public transportation, or healthcare settings, one infected person can expose many others in a short time. The virus travels on air currents and can reach people who were not directly in front of the coughing or sneezing person.

Crowded indoor spaces with poor ventilation make transmission more likely. Measles spreads less readily outdoors, where air movement disperses the droplets more quickly. This is why measles outbreaks often occur in settings where people spend extended time indoors together — classrooms, offices, waiting rooms, and shared housing.

Who is at highest risk of catching measles

People who have not been vaccinated against measles are at the highest risk. Infants under one year old cannot yet receive the measles vaccine and depend on immunity from their mother or protection from those around them. People with weakened immune systems — including those undergoing chemotherapy, those with HIV, or those taking certain medications — are also at greater risk of infection.

Pregnant people who are not immune can pass measles to their unborn baby, which can cause serious complications including premature birth and low birth weight. People born before 1957 in the United States are generally considered immune because measles was widespread then, though some may not have been infected.

When someone with measles is contagious

The contagious period begins four days before the measles rash appears and continues for four days after the rash starts. During the first few days of illness, before the rash is visible, an infected person may have only fever, cough, runny nose, and sore throat — symptoms that look like a common cold. This is why measles can spread widely before anyone realizes it is measles rather than another illness.

Once the characteristic rash appears (usually starting on the face and spreading downward), the person is still contagious for four more days. After that four-day window closes, the person is no longer contagious, even if they still feel sick or the rash has not completely faded.

How vaccination prevents transmission

The measles vaccine works by teaching the immune system to recognize and fight the virus before exposure. Vaccinated people who are exposed to measles either do not become infected at all or develop a much milder illness. Either way, they shed far less virus and are much less likely to transmit it to others.

When a high percentage of a population is vaccinated, the virus cannot spread easily from person to person, even among those who are not vaccinated. This is called herd immunity. For measles, roughly 95 percent of a community needs to be vaccinated to prevent outbreaks. When vaccination rates drop below this threshold, measles can spread rapidly through the unvaccinated portion of the population.

What to do if you have been exposed to measles

If you have been in close contact with someone who has measles and you are not vaccinated or unsure of your vaccination status, contact your doctor or local health department right away. The measles vaccine can prevent infection or reduce severity if given within 72 hours of exposure. Some people may be given an injection of antibodies (called immunoglobulin) instead, which can provide temporary protection.

If you think you have measles symptoms — fever, cough, runny nose, sore throat, or a rash — call your doctor before visiting in person. This allows the office to take precautions to prevent spreading the virus to other patients and staff. Stay home and away from others until your doctor confirms whether you have measles and tells you when it is safe to be around people again.

Frequently Asked Questions

Can you catch measles from someone who is vaccinated?

Vaccinated people rarely catch measles, and if they do, they shed much less virus. The risk of catching measles from a vaccinated person is extremely low. Outbreaks occur primarily among unvaccinated or under-vaccinated populations.

How long does measles stay in the air after someone coughs?

Measles droplets can travel several feet through the air and remain suspended for minutes. The exact time depends on air movement, humidity, and ventilation. This is why measles spreads so easily in enclosed spaces.

Can you get measles twice?

Once you have had measles or been vaccinated, you develop lifelong immunity. You cannot catch measles a second time. This is why the vaccine is so effective — it provides permanent protection.

Is measles contagious before the rash appears?

Yes. Measles is contagious from four days before the rash appears. During this time, symptoms look like a cold or flu, so people often do not realize they have measles and continue their normal activities, spreading the virus to others.