An estimated 60-90 percent of those infected show symptoms, compared to around 20 percent for dengue, which is common in the region. There is no vaccine and the only cure is treatment for the pain and fluid loss.
One consolation for those suffering from the illness is that unlike dengue, which has several variants, people only seem to get chikungunya once.
"The evidence suggests that once you get it and recover, once your immune system clears the virus you are immune for life," Nasci said.
Associated Press writers Ben Fox in Miami; P. Solomon Banda in Fort Collins, Colorado; David McFadden in Kingston, Jamaica; and Carlisle Jno Baptiste in Roseau, Dominica, contributed to this report.