A Frenchman presents measles in Costa Rica.

Costa Rica was proudly released from measles for five years. But all this changed this week thanks to a five-year-old boy and his French parents who went on holiday in Central America on February 18th. The country's health authorities have quarantined the boy and his parents, aged 30 and 35, and are trying to locate anyone the family may have infected.

In particular, the authorities are trying to locate the 311 people who flew from Air France to Costa Rica for 12 hours, according to the local newspaper La Nación. Several hotel workers where the family stayed overnight were vaccinated against the disease.

The French family had gone to a private doctor in Costa Rica to inquire about rashes from their child and then said that other students at his school had had measles, according to the Costa Rica Star.

The last time Costa Rica had a case of measles, it was in 2014, and it was also imported. The last case of measles that appeared in Costa Rica dates back to 2006.

The reason why the French boy and his mother were not vaccinated against measles is not clear, but it is at a time when the World Health Organization has classified "vaccine reluctance" among Top 10 threats to global health this year.