(CNN) UC Davis Health in Sacramento, California, sent letters to about 200 people who visited its emergency department on March 17 to notify them that they may have been exposed to measles.
"You will need to notify your primary health care provider(s) and your child's provider(s) of this possible exposure to discuss your possible risk of infection, vaccination history, and other questions you may have," the letter says.
As for other family members that may also have been in the waiting room, "they will want to notify their doctors as well."
UC Davis Medical Center sent the letters March 25 and 26.
Dr. Dean Blumberg, chief of pediatric infectious diseases at UC Davis Children's Hospital, said in a statement that as soon as measles was suspected in the March 17 case, "we took appropriate precautions in the areas the patient had visited, including an exam room."
"Aside from the one case on March 17, there are no further measles cases or symptomatic patients at UC Davis Medical Center at this time. In an abundance of caution, we sent the attached letter to patients who may have been in the vicinity of that patient."
Measles is a highly contagious disease, according to the US Centers for Disease Control and Prevention. It can spread through coughing and sneezing and can live for up to two hours in the air where an infected person coughed or sneezed.
The symptoms appear about nine to 12 days after a person has been infected. Measles typically begins with mild to moderate fever, cough, runny nose, conjunctivitis or red, watery eyes, the letter said. This is followed by a fever and a rash two or three days later.
People with measles are infectious from four days before to four days after the rash appears. The letter warns that "anyone diagnosed with measles or suspected of having measles should stay home for the four days after the onset of the rash." But infected people without symptoms may still be able to transmit the virus.
The disease can be prevented by the MMR vaccine, which protects against measles, mumps and rubella. Children should get two doses of the MMR vaccine, with the initial dose given at 12 to 15 months of age and a second dose at 4 to 6 years old. The vaccine is about 97% effective at preventing measles, according to recommendations from the CDC.
Rayna Souza's son is one of the people who may have been exposed to measles, confirmed Pamela Wu, UC Davis Health's director of news and media relations.
Souza told CNN affiliate KTXL on Tuesday that her 7-year-old son, Jackson, who is terminally ill, was exposed when she brought him to UC Davis Medical Center because he was having seizures. Souza was later told that during his hospital stay, Jackson had been exposed to measles.
"I felt mortified. My son's already terminal," she said. "I don't have any area to risk any potential anything with him because I'm just, like, walking around him with a plastic bubble just trying to keep him here as long as I can."
Souza said doctors told her that the source was an unvaccinated child who had come in with measles.
Blumberg said that "Jackson was in the room, the emergency department room, where this other patient was seen." Jackson was the next person to be evaluated in the exam room after the person who had measles, "before measles was suspected and the room was closed to be cleaned."
"It was less than an hour separation between them," he said. "So there was potentially still measles virus in the air."
Blumberg said that although Jackson was exposed to the virus, he doesn't believe the child has measles. It's hard for most doctors to identify measles because it's so rare.
"Thankfully, we do not believe he has measles," he said. "The last time our doctors evaluated him on April 2, which is 16 days after exposure, he did not have measles."
But his mother said she's seen all the symptoms, KTXL reported.
Souza said she feels that the negligence of other parents may shorten her son's life. "People making irresponsible decisions for their family and their child need to really, really look and see how that could affect other people, because I still could lose my son."
The second highest number of individual measles cases -- a total of 387 -- has been reported in the United States since the disease was eliminated in 2000. These cases have been confirmed in 15 states from January 1 to March 28, according to the CDC.
Rockland County, New York, is one of the places experiencing an outbreak of measles, which began in October when an unvaccinated person became infected while visiting Israel and came back with the disease. The county has taken an "extremely unusual" step to ban unvaccinated minors from public places in an effort to contain the outbreak.
Around the globe, a total of 98 countries saw an increase in measles cases in 2018, according to UNICEF.
Measles cases increased by 48.1% between 2017 and 2018, based on calculations by the UN's agency for children, using data on 194 countries from the World Health Organization. And 10 countries -- including Brazil, the Philippines and France -- accounted for nearly three-quarters of the total increase in cases in 2018.
According to UNICEF, poor health infrastructure, lack of awareness, civil disorder, complacency and a backlash against vaccinations were factors in some of the recent outbreaks.