Measles is a highly contagious viral infection that spreads through the air and can cause serious complications, especially in young children. It’s caused by the measles virus, a member of the paramyxovirus family, and remains one of the most infectious diseases known to humans. A single infected person can pass the virus to as many as 18 other people in an unvaccinated community.
How Measles Spreads
Measles travels through the air in tiny droplets released when an infected person coughs, sneezes, or breathes. The virus remains active and contagious in the air or on surfaces for up to two hours. That means you can catch measles simply by walking into a room where an infected person was, even if they’ve already left. No direct contact is needed.
A person with measles is contagious from 4 days before the rash appears through 4 days after it shows up. This is part of what makes measles so difficult to contain: people are spreading it before they even know they’re sick. In communities where less than 95% of people are vaccinated, one infected person can easily trigger a chain of new infections.
Symptoms and How They Progress
Measles typically unfolds in stages. After exposure, there’s an incubation period where you feel fine and have no symptoms. Then the first signs appear: high fever, cough, runny nose, and red, watery eyes. These early symptoms look a lot like a bad cold, which is why measles often goes unrecognized at first.
A day or two before the rash breaks out, tiny white spots called Koplik spots may appear inside the mouth, on the inner lining of the cheeks. These spots are unique to measles and serve as an early diagnostic clue. The characteristic red, blotchy rash then spreads from the face downward across the body. Fever often spikes as the rash appears, sometimes reaching 104°F or higher. The entire illness typically lasts about two weeks from start to finish.
Complications That Make Measles Dangerous
For many people, measles resolves on its own. But the virus can cause severe, sometimes fatal complications, particularly in children under 5, adults over 20, pregnant women, and anyone with a weakened immune system.
Pneumonia is the most common cause of death from measles in young children. About 1 in every 20 children with measles develops it. Brain swelling (encephalitis) occurs in roughly 1 out of every 1,000 cases and can lead to permanent brain damage, hearing loss, or intellectual disability.
There’s also an extremely rare but devastating long-term complication called SSPE (subacute sclerosing panencephalitis), a progressive brain disease that can develop years after a measles infection. During the U.S. measles resurgence from 1989 to 1991, an estimated 7 to 11 out of every 100,000 people who contracted measles were at risk for developing SSPE. It is always fatal.
How Measles Is Diagnosed
Doctors often suspect measles based on the combination of fever, the three Cs (cough, coryza, conjunctivitis), and the distinctive rash. But lab confirmation is important, both for the patient and for public health tracking.
Two main tests confirm measles. A blood test can detect specific antibodies that your immune system produces in response to the virus. A blood sample collected within the first few days of the rash can provide strong evidence of infection. If results are unclear early on, a second blood draw 3 to 10 days after symptoms start may be needed. The second test, called RT-PCR, detects the virus’s genetic material directly from a throat swab, nasal swab, or urine sample. This test is most reliable when collected within the first 3 days of the rash, though it can sometimes detect the virus up to 10 to 14 days after rash onset.
Treatment Options
There’s no antiviral drug that kills the measles virus. Treatment focuses on managing symptoms: staying hydrated, controlling fever, and resting. If a bacterial infection like pneumonia develops on top of the measles infection, antibiotics treat that secondary infection.
Vitamin A supplementation is a key part of measles treatment, especially for children. The World Health Organization and the CDC recommend two doses given on consecutive days, with the amount based on the child’s age: younger infants receive smaller doses while children 12 months and older receive higher doses. Vitamin A has been shown to reduce the severity of the disease and lower the risk of death, particularly in children who may already be deficient.
Vaccination and Protection
The MMR vaccine (which protects against measles, mumps, and rubella) is the most effective tool against measles. A single dose is 93% effective. Two doses raise that to 97%. Children typically receive their first dose around 12 to 15 months of age and the second between ages 4 and 6.
Measles requires an unusually high level of community immunity to prevent outbreaks. At least 95% of a population needs to be vaccinated with both doses to keep the virus from gaining a foothold. That threshold is higher than for almost any other vaccine-preventable disease, a direct consequence of how extraordinarily contagious measles is. When vaccination rates dip even slightly below that level, outbreaks follow.
Current Measles Activity
Measles is far from a thing of the past. As of April 30, 2026, the United States had recorded 1,814 confirmed cases for the year, with 24 new outbreaks reported. The vast majority of those cases, about 93%, were linked to outbreaks. South Carolina led the country with 668 confirmed cases, followed by Utah with 431, Texas with 180, Florida with 134, and Arizona with 74.
Global measles activity is also increasing, which raises the risk of travelers bringing the virus back into communities with low vaccination coverage. Nearly all measles cases in the U.S. trace back either to international travel or to contact with someone who traveled abroad. In communities with strong vaccination rates, these imported cases fizzle out. In under-vaccinated pockets, they spark outbreaks that can persist for months.