Dengue is sweeping across the Western Hemisphere in numbers not seen since record-keeping began more than four decades ago, with experts warning that rising temperatures and rapid urbanization are accelerating the pace of infections.

A record more than 4 million cases have been reported throughout the Americas and Caribbean so far this year, surpassing a previous record set in 2019, with officials from the Bahamas to Brazil warning of crowded clinics and new infections daily. More than 2,000 deaths in that region also have been reported.

“This year is the year we’ve been seeing the most dengue in recorded history,” said Thais dos Santos, adviser on surveillance and control of arboviral diseases with the Pan American Health Organization, the regional office of the World Health Organization in the Americas. She noted that record keeping began in 1980. “Vector borne diseases, especially these diseases that are transmitted by mosquitoes … provide us a really good sentinel of what is happening with climate change.”

Poor sanitation and a lack of robust health systems have contributed to a rise in cases, but experts say droughts and floods linked to climate change are causing greater transmission of the virus, with stored water and heavy rains attracting mosquitoes.