Measles is caused by a virus that spreads through the air

Measles is caused by infection with the measles virus, which travels through the air when an infected person coughs or sneezes. The virus enters your body through your nose or mouth, then multiplies in your lungs and spreads to other parts of your body. You can catch measles from someone who is sick, even if you are not in the same room — the virus can linger in the air or on surfaces for up to two hours after an infected person has left.

The measles virus is one of the most contagious viruses known. If you are not protected by vaccination or prior infection, your chance of catching measles after exposure to someone who has it is about 9 in 10. This is why measles spreads so quickly through communities where vaccination rates are low.

Key Takeaways

  • Measles spreads through respiratory droplets when an infected person coughs or sneezes, and the virus can remain in the air for up to two hours.
  • You can catch measles from someone who is contagious even before they develop the characteristic rash, making it easy to spread without knowing.
  • The measles virus is extremely contagious — about 90 percent of unvaccinated people who are exposed will become infected.
  • Vaccination or prior measles infection provides lifelong protection against catching the disease again.

How the virus enters and spreads through your body

When you breathe in air containing the measles virus, it lands on the cells lining your respiratory tract — the passages in your nose, throat, and lungs. The virus attaches to these cells and begins to multiply. Within a few days, the virus spreads into your bloodstream and travels throughout your body, infecting cells in your lungs, skin, immune system, and other organs.

This is why measles causes symptoms in so many different parts of your body at once. The high fever, cough, and runny nose happen because the virus is actively multiplying in your lungs and airways. The characteristic rash appears later because the virus has spread to your skin cells.

When an infected person is most contagious

A person with measles is contagious from about four days before the rash appears until four days after it appears. This timing is important because people are often most contagious before they know they have measles — when they think they just have a cold or flu. During this early period, they may go to work, school, or public places and spread the virus to others.

The first symptoms of measles are fever, cough, runny nose, and red eyes. The rash typically appears three to four days after these initial symptoms start. By the time someone realizes they have measles and stays home, they may have already exposed many other people.

Why some people are protected and others are not

If you have been vaccinated against measles with the MMR vaccine (which protects against measles, mumps, and rubella), your immune system has learned to recognize and fight the measles virus. When you are exposed to the virus, your body destroys it before it can cause infection. The MMR vaccine is about 97 percent effective at preventing measles after two doses.

If you have had measles before, your body also remembers the virus and can fight it off if you are exposed again. This immunity typically lasts your entire life. People who are not vaccinated and have never had measles have no protection, which is why the virus spreads rapidly in unvaccinated communities.

Conditions that make measles spread faster

Measles spreads more easily in crowded environments where people are close together for extended periods. Schools, childcare centers, hospitals, and public transportation are common places where measles outbreaks occur. Poor ventilation makes spread more likely because the virus remains in the air longer.

Measles also spreads more readily in communities with lower vaccination rates. When fewer people are vaccinated, the virus has more people to infect and more opportunities to spread. Even one unvaccinated person can start an outbreak if they travel to a new area or attend a gathering with other unvaccinated people.

Why measles is different from other respiratory viruses

Many viruses spread through coughs and sneezes — cold viruses, flu viruses, and others. Measles is more contagious than most of these because the virus is hardier and survives longer outside the body. A single infected person with measles will typically infect 12 to 18 other people in an unvaccinated population, compared to 1 to 3 people for the seasonal flu.

The measles virus also has a longer incubation period — the time between infection and the appearance of symptoms — which is usually 10 to 14 days. This means infected people can travel and spread the virus to new areas before they realize they are sick. This is one reason why measles outbreaks can spread across regions and countries.

What happens if you are exposed but not infected

If you are vaccinated and exposed to measles, the vaccine-induced immunity in your body will recognize the virus and destroy it. You will not develop measles, and you will not spread it to others. This is called sterilizing immunity — the virus cannot establish an infection at all.

If you are unvaccinated and exposed, whether you become infected depends partly on the amount of virus you were exposed to and the health of your immune system. People with weakened immune systems — from conditions like HIV, cancer treatment, or certain medications — are at higher risk of infection even after exposure to smaller amounts of virus.

Frequently Asked Questions

Can you catch measles twice?

No. Once you have had measles or been vaccinated against it, you are protected for life. Your immune system remembers the virus and will fight it off if you are exposed again. Reinfection is extremely rare.

How long does it take to get sick after being exposed to measles?

The incubation period is usually 10 to 14 days, though it can range from 7 to 21 days. This means you may not develop symptoms for up to three weeks after exposure, and you could spread the virus to others during this time without knowing you are infected.

Can you catch measles from someone who is vaccinated?

No. Vaccinated people do not develop measles and do not shed the virus. The vaccine prevents infection entirely, so vaccinated people cannot spread measles to others.

Is measles contagious through touching or sharing food?

Measles spreads through respiratory droplets in the air, not through touch or shared food. However, the virus can land on surfaces and remain alive for up to two hours, so touching a contaminated surface and then touching your face could potentially transmit the virus.

Why do babies sometimes get measles even though their mothers were vaccinated?

Vaccinated mothers pass antibodies to their babies through breast milk, which provides temporary protection in the first few months of life. This protection fades by around 12 months, which is why the MMR vaccine is first given at 12 to 15 months of age. Babies born to unvaccinated mothers have no protection from birth.