  Multi-strain diseases are diseases that consist of several strains, or
serotypes. The serotypes may interact by antibody-dependent enhancement (ADE),
in which infection with a single serotype is asymptomatic, but infection with a
second serotype leads to serious illness accompanied by greater infectivity. It
has been observed from serotype data of dengue hemorrhagic fever that outbreaks
of the four serotypes occur asynchronously. Both autonomous and seasonally
driven outbreaks were studied in a model containing ADE. For sufficiently small
ADE, the number of infectives of each serotype synchronizes, with outbreaks
occurring in phase. When the ADE increases past a threshold, the system becomes
chaotic, and infectives of each serotype desynchronize. However, certain
groupings of the primary and second ary infectives remain synchronized even in
the chaotic regime.
