The World Health Organization on Tuesday said tens of thousands of peopleare believed to be infected with dengue fever on La Réunion.The island – a French territory – is a popular vacation destination in the Indian Ocean.Dengue is a set of viruses found in tropical and sub-tropical parts of the world.Humans get dengue from being bitten by mosquitos infected with the virus.Health officials on the island told the WHO of the disease outbreak in March 2018.About 50,000 probable cases of the diseasewere reported between 2018 and April of this year.That includes 22,000 so far in 2019, the health agency said in a statement.The WHO described the rise of confirmedand probable cases reported last year as unprecedented.There is no treatment. But with early discovery and good medical care,fewer than one percent of people infected die from the disease.The WHO said 14 people have died in the Réunion outbreak since 2018.The territory has a population of 866,500.Around the world, the number of dengue cases dropped from 2017 to 2018.But there has been a huge increase in 2019, especially in Vietnam,Australia, Cambodia, China, Laos, Malaysia, the Philippines and Singapore.About one in four people infected with dengue will get sick.Of those who do get sick, the symptoms can be mild to severe.Severe dengue can be deadly and often requires hospitalization.Only a small percentage of people get severe dengue.Mild dengue can be mistaken for other sicknesses, such as influenza.Symptoms often begin four to seven days after being bittenand commonly last two days to one week.They can include high fever, severe headaches, fatigue,vomiting, and, in severe cases, bleeding.The first dengue vaccine, Dengvaxia, was approved in 2015for use in Mexico, Brazil and the Philippines.Several other countries followed in 2016.The U.S. Food and Drug Administration approved Dengvaxia in 2019.The WHO says the vaccine should not be given to peoplewho have not had an earlier dengue infection.The World Health Organization notes that dengue feveris now common in over 100 countries.Forty percent of the world's population lives in an area at risk for dengue.