  Recent work has found that the behavior of an individual can be altered when
infected by a parasite. Here we explore the question: under what conditions, in
principle, can a general parasitic infection control system-wide social
behaviors? We analyze fixed points and hysteresis effects under the Master
Equation, with transitions between two behaviors given two different
subpopulations, healthy vs. parasitically-infected, within a population which
is kept fixed overall. The key model choices are: (i) the internal opinion of
infected humans may differ from that of the healthy population, (ii) the extent
that interaction drives behavioral changes may also differ, and (iii) indirect
interactions are most important. We find that the socio-configuration can be
controlled by the parasitically-infected population, under some conditions,
even if the healthy population is the majority and of opposite opinion.
