Health officials in Philadelphia have confirmed a measles outbreak in the city, and it all stems from a child who contracted the virus after being out of the country continued going to daycare. So far, there have been eight reported cases — all but one from Philadelphia — with officials urging residents who are not vaccinated to receive one ASAP.

According to the Philadelphia Inquirer, the outbreak began when an infant was hospitalized with the virus at the Children’s Hospital of Philadelphia in mid-December. The infant had been in a country where measles is “regularly occurring,” though it’s not clear which country they were in. The virus then spread to two other unvaccinated children in the hospital: an infant too young to have been vaccinated and an eligible child whose parents had refused to vaccinate, according to the outlet. The older child’s parents also refused medication that can prevent infection following exposure.

Now, the outlet reports that an infected child was sent to an area daycare despite quarantine instructions, causing an outbreak of four other infections. So far, it seems that the cases are contained to the children’s hospital and the daycare center, but health officials are urging residents who are not vaccinated to get the MMR vaccine — which protects against measles, mumps, and rubella — as well as remain isolated for 21 days if they were exposed.

None of the infected individuals had prior immunity to measles — which means they’d either never been vaccinated or had a previous infection — making vaccination and adhering to quarantine guidelines if exposed particularly important. Health officials note that while 93 percent of the city’s children are vaccinated, measles is a “dangerous virus” to those who are not.

According to the U.S. Centers for Disease Control and Prevention (CDC), measles is a highly contagious virus that spreads through the air or on surfaces when an infected person sneezes, coughs, or breathes. It can survive in the air for two hours after an infected person leaves an area. The World Health Organization (WHO) notes that symptoms include a high fever, cough, runny nose, and full-body rash. It can impact anyone but is especially dangerous for children under 5, pregnant people, and those with weakened immune systems.

NBC News reports that roughly 1 in 5 unvaccinated people with measles ends up hospitalized, and the virus is fatal to 1 to 3 out of every 1,000 children with measles due to severe complications such as pneumonia or swelling of the brain. High vaccine rates helped eliminate measles outbreaks in the U.S. up until the year 2000, but there have since been occasional outbreaks originating from other countries. 

Dr. Paul Offit, director of the Vaccine Education Center at Children’s Hospital of Philadelphia, told NBC News that the recent outbreaks are likely a result of declining vaccination rates. “Measles is the most contagious of the vaccine-preventable diseases, so when you lower immunization rates, that’s the first disease to come back,” he said.

It’s unclear if any of the infected individuals in Philadelphia have experienced severe illness, but here’s hoping that the outbreak will be a fresh reminder that staying vaccinated against communicable illness remains the number one way to stay safe and to protect others.