A 23-year-old medical student on the Caribbean island of Dominica presents at the Student Health Clinic complaining of an increasingly severe headache and back and bone pain. Yesterday she was nauseated and vomited several times during the night. She has a 39.5°C/103°F fever, which appeared suddenly, and a generalized rash that blanches under pressure. She had been hiking in the rainforest 1 week earlier and was particularly bothered by mosquitoes at that time. The most likely infectious agent causing her symptoms is