The age distribution of DHF/DSS in infants explained as a function of dengue antibodies that protect at birth, but as they wane they enhance dengue infections and finally disappear; at 12 -18 months, a period of very low DHF/DSS attack rates, infants for the first time are susceptible to unmodified primary dengue infections. (Halstead SB et al. Yale J Biol Med 42:311-328,1970; Halstead SB Amer J Epidemiol 114:632-648,1981; Kliks S et al AJTMH 38:441-419, 1988)