"A patient treated at Texas Children's Hospital West Campus tested positive for measles. This is a highly contagious, vaccine-preventable infection. We know vaccination is the best protection against measles," the hospital said in a statement.
"We were also made aware that one of our nurses posted protected health information regarding a patient on social media," the hospital added.
Treatment, preventative care and vaccinations for measles across the world
Treatment, preventative care and vaccinations for measles across the world
A child receives a vaccination against measles by a family physician on April 16, 2018 in the Romanian capital, Bucharest.
Measles still claims young lives in Romania, where nearly 40 children have died in an outbreak that many blame on parents being misled by scare stories that vaccinating them is dangerous. Some 12,000 people have contracted measles since late 2016 in the European Union's second-poorest country, 46 of them died. Among the dead, 39 were children under the age of three who had not been vaccinated, making Romania one of the worst affected countries in the ongoing measles outbreak in Europe. / AFP PHOTO / Daniel MIHAILESCU (Photo credit should read DANIEL MIHAILESCU/AFP/Getty Images)
A family physician prepares a measles vaccine during a consultation on April 16, 2018 in the Romanian capital, Bucharest. - Measles still claims young lives in Romania, where nearly 40 children have died in an outbreak that many blame on parents being misled by scare stories that vaccinating them is dangerous. Some 12,000 people have contracted measles since late 2016 in the European Union's second-poorest country, 46 of them died. Among the dead, 39 were children under the age of three who had not been vaccinated, making Romania one of the worst affected countries in the ongoing measles outbreak in Europe. (Photo by Daniel MIHAILESCU / AFP) (Photo credit should read DANIEL MIHAILESCU/AFP/Getty Images)
A nurse was seen vaccinating school children at her school in Lhokseumawe, August 4, 2018, Aceh, Indonesia. Vaccination Measles-Rubella (MR) for Indonesian school children is part of the Indonesian government's health program to provide physical immunity from two dangerous diseases, Rubella and Measles in Indonesian children. (Photo by Fachrul Reza/NurPhoto via Getty Images)
Artists wearing superhero costumes comfort an elementary student while she receives a German Measles, also known as Rubella, vaccination in Pekanbaru, Riau province on August 1, 2018 as part of the Indonesian governments health program to combat childhood diseases. (Photo by WAHYUDI / AFP) (Photo credit should read WAHYUDI/AFP/Getty Images)
ALEPPO, SYRIA - APRIL 17 : A Syrian child is vaccinated during the measles campaign in Al-Bab district of Aleppo, Syria on April 17, 2018. Kids fled from Eastern Ghouta's Douma after Assad regime forces' suspected chemical attack on April 7, 2018 receive measles vaccine Al-Bab district of Aleppo. (Photo by Omer Alven/Anadolu Agency/Getty Images)
A Venezuelan woman is vaccinated against measles in Cucuta, Colombia, at the international brigde Simon Bolivar on the border with Venezuela, on March 21, 2018. / AFP PHOTO / Schneyder Mendoza (Photo credit should read SCHNEYDER MENDOZA/AFP/Getty Images)
Venezuelan, Yan Manuel, receives a free measles vaccine given by a volunteer, after showing his identity card at the Pacaraima border control, Roraima State, Brazil August 19, 2018. REUTERS/Nacho Doce
A member of the Free Revolutionary Police carries out an inspection on a measles vaccine in the southern Idlib countryside September 17, 2014. Fifteen children died after being vaccinated against measles in northern Syria, resulting in the programme being halted, aid workers said on Wednesday, a tragedy likely to damage trust in health services in opposition-held areas. The Free Revolutionary Police is a newly launched unarmed police movement that works in areas controlled by the Free Syrian Army to solve the day-to-day problems in Idlib, the group said. Picture taken September 17, 2014. REUTERS/Khalil Ashawi (SYRIA - Tags: CIVIL UNREST CONFLICT HEALTH)
A girl receives anti-measles vaccination drops at a health centre in BASECO compound in Tondo, Manila September 3, 2014. Philippine President Benigno Aquino said on September 1, 2014 between 11 to 13 million people in the country are at risk from measles, polio and rubella (German measles), and asked the public to cooperate in eradicating the preventable diseases, during Monday's launch of a mass national vaccination campaign against measles and polio, reported local media. REUTERS/Erik De Castro (PHILIPPINES - Tags: HEALTH SOCIETY)
A vial of measles, mumps and rubella vaccine and an information sheet is seen at Boston Children's Hospital in Boston, Massachusetts February 26, 2015. REUTERS/Brian Snyder (UNITED STATES - Tags: HEALTH)
Rowan Walters, aged four, is held by her mother as she given an MMR injection at the Paediatric Outpatients department at Morriston Hospital in Swansea, south Wales April 6, 2013. Following an increase in the number of confirmed cases of a measles in south Wales, parents in the area were urged to vaccinate their children, and hospitals in the region opened drop-in clinics on Saturday, local media reported. REUTERS/Rebecca Naden (BRITAIN - Tags: SOCIETY HEALTH)
A health worker in Ciudad Juarez at the Mexican border crossing with El Paso, Texas, hands out leaflets to passing motorists and people as part of a measles prevention programme, February 16, 2015. California public health officials have confirmed three more cases of measles in an outbreak that began in late December, bringing to 113 the total number of people believed to have been infected in the state. Health officials in Arizona, where seven cases of measles have been documented, said the outbreak would likely be considered over in that state if no further infections were reported over the weekend. Across the United States, more than 150 people have been diagnosed with measles, many of them linked to an outbreak that authorities believe began when an infected person from out of the country visited Disneyland in late December. REUTERS/Jose Luis Gonzalez (MEXICO - Tags: SOCIETY HEALTH IMMIGRATION)
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"We take these matters very seriously, as the privacy and well-being of our patients is always a top priority. After an internal investigation, this individual is no longer with the organization."
The hospital did not identify the nurse. A Facebook user who alerted Texas Children's to the post said she was shocked by what the nurse had said, and copied a screenshot of it to the Texas Children's Facebook page.
"The kid was super sick. Sick enough to be admitted to the ICU and he looked miserable," the nurse posted to an anti-vaccine Facebook page. But she said she still opposed vaccines.
This shocked Dr. David Persse, director of the Houston Health Department. "She is one of the few people who has seen firsthand how devastating these diseases can be, and she has still taken this position against vaccines," Persse said in an interview.
He pointed out that many studies have established the safety of vaccines, and that medical professionals should be educated about the facts.
"You have a greater risk of being struck by lightning than you do of having a serious adverse event to a vaccine," Persse said.
Measles has been eliminated in the United States through vaccination, but every year, cases are imported from other countries. People who are not vaccinated or who are incompletely vaccinated can become infected and carry the virus with them. It will spread if there are pockets of other unvaccinated people.
Persse said his department is helping track everyone that the child might have been in contact with while infectious. Measles is one of the most infectious viruses known, and people can spread it before they begin showing symptoms.
Measles killed nearly 90,000 people globally in 2016, according to the World Health Organization. Nine out of 10 people who have not been vaccinated will develop an infection if they are exposed to someone with the virus, and can even become infected if they walk into a room where an infected person has been.
So when a case of measles is identified, health officials work to track down anyone who could have been exposed.
"We got lucky this time," Persse said. He said the child was not in contact with many people other than his family before he was isolated in the hospital.
The Facebook user who revealed the nurse's posts said she used an alias on social media and did not wish to be identified for fear of retaliation by members of the anti-vaccine movement, who often attack medical professionals when they speak out about the safety of vaccines
"They will stalk us. They will threaten us. They will call child protective services on us," she told NBC News.