Maricopa County public health officials say they are confronting the largest measles outbreak the region has seen in decades, and they are urging residents to get vaccinated.

Officials have linked the outbreak to the city of Mesa and are coordinating the response across Maricopa County. The announcement carries no specific case counts, and describing the outbreak as the most significant in decades suggests this is far beyond the routine single-case investigations health departments handle most years.

Measles is among the most contagious viruses known. It travels through the air when an infected person coughs or sneezes, and it can remain in the air for up to two hours after that person leaves a room. Anyone who is not immune has a very high chance of catching it after sharing a space with someone who is infectious. Symptoms typically start with a fever, cough, runny nose, and red, watery eyes, followed by the characteristic rash that spreads over the body.

Most healthy people recover from measles within a couple of weeks, but the disease can be severe. Complications include pneumonia, ear infections that can cause permanent hearing loss, and swelling of the brain. The risk is highest for infants, pregnant women, and people with weakened immune systems. In rare cases, measles is fatal.

That is why the response from officials centers on vaccination. The measles, mumps, and rubella vaccine, known as MMR, protects people through two doses. Children usually receive the first around their first birthday and the second before starting school. Two doses provide lasting immunity in the vast majority of recipients. The MMR vaccine has a decades-long safety record, and it is the most effective tool available for stopping the disease from spreading.

The vaccination push reflects how measles behaves in a community. A large share of immune residents makes it hard for the virus to spread, and even people who cannot be vaccinated, such as infants too young for the shot or people with certain medical conditions, are shielded by the immunity of those around them. Dropping vaccination rates weaken that shield, and outbreaks tend to begin in pockets of unvaccinated people before spilling into the wider community.

For Mesa residents and everyone in Maricopa County, the practical steps are clear. Check vaccination records for yourself and your children, and make sure MMR shots are up to date. Adults who were vaccinated as children are generally still protected, but anyone who missed a dose should catch up. People who are unsure of their status can ask a health care provider to check.

Anyone with symptoms such as fever and rash should not walk straight into a waiting room or urgent care clinic. Measles is contagious enough that one infected person can expose an entire room full of patients and staff. Call ahead instead, describe your symptoms, and follow the instructions you receive about where to go and how to avoid spreading the virus.

The advice matters most for families with young children. Infants do not get their first MMR dose until around their first birthday, leaving them vulnerable during their first year. That gap is a major reason community immunity is so important: the protection of everyone around the youngest residents is what keeps them safe.

The official announcement does not say how the outbreak began or how much further it will reach. Measles outbreaks in the United States frequently trace back to international travel, and a single infected traveler can ignite an outbreak that lasts months in a community with low vaccination coverage. The longer a region goes without measles, the larger the pool of people who have never been infected and never vaccinated. County officials have not confirmed the source of this outbreak, and tracking it down is part of the work now under way.

The scale is what makes this event different. Health departments usually contain measles with a mix of isolation and contact tracing, identifying everyone who was exposed and making sure they are not contagious during the incubation period. An outbreak described as the largest in decades means the containment playbook has had to stretch, and the virus has had time to move through the community.

Expect more cases in the weeks ahead. Measles has an incubation period of one to two weeks, so a person exposed today will not show symptoms for days. Even if the outbreak is peaking now, new cases will keep appearing as people exposed in recent weeks become sick. Officials will keep tracking cases, identifying exposure sites, and pressing the vaccination message.

The path out of this outbreak is the same as the path into it, reversed. Measles outbreaks are not random events. They grow out of vaccination gaps, and closing those gaps is what stops the spread. The vaccine is safe, widely available, and the one step that protects both the person who gets it and the people around them.

For anyone in Mesa or the rest of Maricopa County who is not vaccinated, the time to act is now, not after a case turns up in your household or your child's school. Measles does not wait for a convenient moment.