Measles is a highly contagious viral infection that spreads through the air and causes fever, cough, and a distinctive full-body rash. It remains one of the most infectious diseases known to humans, with one sick person capable of infecting 12 to 18 others in an unvaccinated population. While vaccination has dramatically reduced cases worldwide, measles still causes serious outbreaks, including more than 2,200 confirmed cases in the United States in 2025 alone.
How Measles Spreads
The measles virus travels in tiny respiratory droplets released when an infected person coughs, sneezes, or even breathes. What makes it exceptionally dangerous is that these droplets can hang in the air for up to two hours after the person has left the room. You can catch measles simply by walking into a space where someone with the virus was present, without ever coming into direct contact with them.
A person with measles is contagious for four days before the rash appears and four days after. That pre-rash window is particularly problematic because people don’t yet know they’re sick, so they continue going about their daily lives, potentially exposing dozens of others.
Symptoms and How They Progress
Measles doesn’t start with the rash most people associate with the disease. It begins with what looks like a bad cold: high fever, cough, runny nose, and red, watery eyes. This initial phase typically lasts two to four days and is easy to mistake for the flu or another respiratory illness.
Before the rash appears, tiny white spots may develop inside the mouth, on the inner lining of the cheeks. These are called Koplik spots, and they’re a telltale sign of measles that no other common illness produces. They usually show up a day or two before the rash and fade once it arrives.
The rash itself typically starts on the face and spreads downward over the body across three to four days. It consists of flat red spots that can merge together. Fever often spikes when the rash appears, sometimes reaching 104°F or higher. Most people begin recovering once the rash fades, with the whole illness lasting roughly two to three weeks from exposure to resolution.
Complications to Watch For
Measles is not a mild childhood illness. It carries real risks, particularly for young children, pregnant women, and anyone with a weakened immune system. About 1 in 10 children with measles develops an ear infection. Roughly 1 in 20 develops pneumonia, which is the most common cause of death from measles in young children.
Brain swelling (encephalitis) occurs in about 1 out of every 1,000 children who get measles. This can lead to permanent brain damage, hearing loss, or intellectual disability. An even rarer complication called SSPE (subacute sclerosing panencephalitis) can emerge years after the initial infection. It’s a fatal disease of the nervous system, estimated to affect 7 to 11 out of every 100,000 people who contract measles.
Immune Amnesia
One of the most striking dangers of measles is something researchers have identified in recent years: it can wipe out your immune system’s memory. A 2019 study published in Science found that measles eliminated between 11% and 73% of a person’s existing protective antibodies, the ones built up from past infections and vaccinations against other diseases. This means that after recovering from measles, your body may “forget” how to fight off illnesses it previously knew how to handle, like the flu or strep. Researchers found that people slowly rebuilt this protection through natural re-exposure to pathogens over time, but the vulnerability window is significant. Importantly, this immune amnesia was not seen in vaccinated children.
How Measles Is Treated
There’s no antiviral medication that targets measles directly. Treatment focuses on managing symptoms: staying hydrated, controlling fever, and resting. Children with measles are sometimes given vitamin A supplements, which the World Health Organization recommends in areas where vitamin A deficiency is common. This has been shown to reduce the severity of the illness and lower the risk of complications.
If pneumonia or another bacterial infection develops as a secondary complication, antibiotics can treat those specific infections, but not the measles virus itself. Most otherwise healthy people recover fully within two to three weeks.
Vaccination and Protection
The MMR vaccine (which covers measles, mumps, and rubella) is the primary defense against the disease. A single dose is 93% effective at preventing measles. Two doses raise that to 97%. Children in the U.S. typically receive the first dose between 12 and 15 months of age and the second between 4 and 6 years old.
If you’re unsure of your vaccination status, a blood test can check for measles antibodies. Adults born before 1957 are generally considered immune because measles was so widespread that virtually everyone was exposed. Those born after 1957 who haven’t been vaccinated or can’t confirm two doses may want to get vaccinated, especially before international travel.
Recent Outbreaks in the U.S.
Measles cases in the United States surged dramatically in 2025, with 2,281 confirmed cases and 50 outbreaks reported, compared to just 285 cases and 16 outbreaks in 2024. Ninety percent of the 2025 cases were linked to outbreaks rather than isolated travel-related infections, which signals sustained community transmission. The vast majority of cases in recent U.S. outbreaks have occurred in unvaccinated individuals or in communities with lower vaccination rates, reinforcing how quickly the virus exploits gaps in immunity.