What is dengue fever, the cause of Puerto Rico’s public health emergency?

Puerto Rico Health Secretary Carlos Mellado declared a public health emergency last week in response to a rise in cases of dengue fever. While Mellado said “cases have exceeded historical figures” this year, Puerto Rico is not alone in dealing with the virus.

Other places with hot and humid climates, such as Brazil and Peru, are also facing outbreaks. Across Latin America, cases of infection have surged to 3.5 million — causing over 1,000 deaths. According to Pan American Health Organization director Jarbas Barbosa, it represents triple the number of cases experienced this time last year, which was already a record year.

The Atlanta-based Centers for Disease Control and Prevention reported that dengue fever is spread through the bite of infected mosquitos and that roughly half of the world’s population lives somewhere the virus can spread.

It first became a worldwide issue in the 1960s and has become common in popular tourist destinations, such as the Caribbean. A vaccine is available for children 9-16 years old, but it is not approved for U.S. travelers visiting in areas where the virus is common. And there is no medicine to threat dengue fever directly.

Up to 400 million people are infected each year, leading to around 40,000 deaths and 100 million illnesses. Symptoms can range from mild to severe, and a quarter of those infected often get sick.

The most common symptom is fever, along with nausea, vomiting, rashes, eye pain, muscle pain, joint pain and bone pain. Symptoms generally last anywhere from two to seven days. Around 5% of those that get sick from the virus, however, develop severe symptoms.

Severe dengue can lead to shock, internal bleeding and death. Infants, pregnant women and those that have been previously infected are all at higher risk of developing severe symptoms.