In order to analyze the effects of acquired immunity and the contact pattern with infested water on the overall transmission of schistosomiasis, a semi-stochastic model is proposed. The model's assumptions are the simplest possible to enhance the differences between two hypotheses. With respect to the human host, it is assumed the mounting of an immune response after elapsing a fixed period of time L from the first infection, which is partially effective and never lost. With respect to the contact pattern with infested water, it is assumed a decreasing age-related function. Both models are compared to a purely random model, which is taken as the basic model.