Decadal average age distribution of hospitalized DHF in Bangkok during the 1970s, 80s and 90s. Two features are notable: 1) bimodal distribution, with one mode among infants; 2) modal age of hospitalization steadily moves up (from 5 to 9 years). DHF in infants accompanies primary infections while DHF in children 1 year and older accompanies secondary dengue infections.