Antibody-dependent enhancement (ADE) of infection is one of the hypothetical mechanisms underlying the increased disease severity from DF to DHF [10]. A secondary infection with the serotype responsible for the primary infection is protective, whereas a secondary infection with a different serotype produces 15–80 times more cases of DHF than are produced by the primary infection [11]. Therefore, a preexisting heterotypic immune factor(s) may cause DHF. Although various immune-associated factors have been proposed, their involvement is still contentious [2] and [12]. However, most researchers agree that higher viremia levels increase disease severity [13]. Higher viremia can be explained by the ADE hypothesis. Briefly, a preexisting nonneutralizing cross-reactive antibody (the “enhancing antibody”) increases the number of infected monocytes/macrophages in an FcγR-mediated manner [14].