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

You catch measles by inhaling virus particles that an infected person releases into the air. When someone with measles coughs or sneezes, they send out tiny droplets containing the virus. If you breathe in those droplets, the virus can infect your respiratory tract and begin to multiply. This is why measles spreads so easily in enclosed spaces where people spend time together — schools, offices, waiting rooms, public transit.

The virus can also survive on surfaces for a short time. If an infected person touches their nose or mouth and then touches a doorknob, handrail, or other object, the virus transfers to that surface. If you touch that surface and then touch your face, nose, or mouth, you can become infected. This route is less common than breathing in droplets, but it does happen.

Measles is one of the most contagious diseases known. A single infected person will transmit the virus to about 12 to 18 people in an unvaccinated group, compared to 1 to 2 people for flu. This high transmission rate means that if you are around someone with measles and you are not vaccinated or immune, your risk of infection is substantial.

Key Takeaways

  • Measles spreads when you breathe in droplets from an infected person's cough or sneeze, making it airborne and highly contagious.
  • You can also catch measles by touching a contaminated surface and then touching your face, though this is less common than airborne transmission.
  • An infected person can spread measles to others starting four days before their rash appears and continuing for four days after the rash begins.
  • The MMR vaccine prevents measles infection in about 97 percent of people who receive both doses, making vaccination the most effective way to avoid catching it.

When an infected person is contagious

Someone with measles can spread the virus to others before they know they have it. The contagious period begins about four days before the rash appears — when the person may only feel like they have a cold or flu. At this early stage, they might not realize they have measles, so they continue their normal activities and expose others without knowing it.

The contagious period continues for about four days after the rash first appears. After that point, the person's body has begun to control the infection and they shed far fewer virus particles. This means the window of highest risk is roughly eight days total, centered around when the rash becomes visible.

During the contagious period, the virus is present in respiratory secretions — saliva, mucus, and the tiny droplets released when breathing. It is also present in urine and other body fluids, though respiratory transmission is the main route of spread.

Who is most at risk of catching measles

People who have not received the MMR vaccine are at highest risk. The vaccine provides immunity in about 97 percent of people who receive both doses on schedule. One dose provides about 93 percent protection, which is why two doses are recommended. If you have not been vaccinated or do not know your vaccination status, you are vulnerable to infection if exposed.

Infants under one year old cannot receive the MMR vaccine yet, so they depend on immunity from their mother (if she was vaccinated or had measles) and on the vaccination of people around them. Infants with waning maternal immunity are at particular risk for severe measles.

People with weakened immune systems — from HIV, certain medications, or other conditions — may not develop full immunity even after vaccination. They are also at higher risk if exposed. Pregnant people who are not immune should avoid exposure, since measles during pregnancy increases the risk of miscarriage and premature birth.

Exposure in healthcare and travel settings

Healthcare workers face higher exposure risk because they encounter patients with measles before diagnosis is confirmed. A patient may arrive at an emergency room or clinic with fever and rash, and measles may not be immediately recognized. Healthcare workers without immunity who care for that patient during the contagious period can become infected.

International travel increases exposure risk in regions where measles is more common. Many countries have lower vaccination rates than the United States, so measles circulates more widely. If you travel to such a region without immunity, your risk of exposure is higher. You can also bring measles back home and expose others.

Outbreaks in schools and childcare settings spread rapidly because children spend hours in close proximity. A single unvaccinated child with measles can trigger an outbreak affecting dozens of other children and staff members.

What happens after exposure

If you have been 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 may feel fine and have no idea you are infected. You can still spread the virus to others during this time, even though you have no symptoms.

The first symptoms are usually fever, cough, runny nose, and red eyes — symptoms that look like a cold or flu. Two to three days later, a rash appears, usually starting on the face and spreading downward. By the time the rash appears, you have already been contagious for several days.

If you think you have been exposed to measles, contact your doctor or local health department. They can assess your vaccination status and immunity. If you are not immune and were recently exposed, a post-exposure dose of the MMR vaccine given within 72 hours of exposure may prevent infection or reduce severity.

Immunity from vaccination versus natural infection

The MMR vaccine creates immunity that lasts a lifetime for most people. After two doses, you are protected against measles in about 97 percent of cases. This protection does not fade over time the way immunity from some other vaccines does.

Natural infection — actually having measles — also creates lifelong immunity. Once you recover from measles, your body has learned to recognize and fight the virus, and you will not catch it again. However, the cost of natural infection is high: measles causes high fever, severe rash, and can lead to pneumonia, encephalitis, and other serious complications. Vaccination provides the same immunity without those risks.

Some people born before 1957 are assumed to be immune because measles was so common that most people caught it naturally. However, this assumption is not always correct, and older adults who are unsure of their immunity can request a blood test to check.

Frequently Asked Questions

Can you catch measles from someone who is vaccinated?

No. A vaccinated person who is immune cannot catch measles and therefore cannot spread it. Measles spreads only from people who are infected. In rare cases, a vaccinated person may have a vaccine failure and still become infected, but this happens in only about 3 percent of vaccinated people.

How long does the virus survive on surfaces?

Measles virus can survive on surfaces for up to two hours, though it loses infectivity over time. Surfaces in a room where an infected person spent time may pose a risk for a few hours after they leave. Standard cleaning does not require special disinfectants — regular soap and water or household cleaners work.

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

Contact your doctor or local health department right away. Tell them about the exposure so they can advise you based on your vaccination status and immunity. If you are not immune and were exposed within the last 72 hours, a dose of MMR vaccine may help prevent infection. Avoid contact with others, especially infants and pregnant people, until your doctor says it is safe.

Can you catch measles twice?

No. Once you have had measles or been vaccinated against it, you develop immunity that lasts your entire life. You cannot catch measles a second time. This is why measles outbreaks occur only in populations with low vaccination rates.

Is measles contagious before the rash appears?

Yes. Measles is contagious starting about four days before the rash appears, when symptoms feel like a mild cold. This is why measles spreads so easily — people do not realize they have it and continue their normal activities, exposing others. The rash is actually a sign that the immune system is responding, not the start of contagiousness.