Parasite infection poses a serious menace to sociality, because living in a group could mediate secondary transmission of parasites. In particular, when host populations exhibit viscosity (i.e., social interactions occur locally due to limited dispersal), horizontal transmission takes place among closely related individuals, which could result in the decreases in inclusive fitness. Therefore, social animals have developed several behavioral mechanisms for reducing the risk of horizontal transmission among sib. In this presentation, I briefly introduce sickness behaviors, namely dispersal, reduced transmissibility, and suicide, and demonstrate mathematical models to predict the evolutionary consequences (i.e., evolutionarily stable strategy) for such traits. By evaluating the inclusive fitness cost/benefit for such strategies, I show that the models offer testable predictions for the direction of selection on sickness behaviors, and discuss the effects of population viscosity on ESS.