System Dynamics provides a suitable framework for modeling diseases that can bring together biological, epidemiological, clinical, and  public health issues in a manageable way.  In this paper, we discuss models for the development and transmission of  parasite-borne diseases which involve a primary host (e.g. a human), a vector(e.g. a mosquito), and a parasite .  The models discussed here are being applied to different diseases; this paper will refer primarily to results for filaria.   We first examine models  for the  biology of the disease in a vector and in a host, and for disease transmission between two humans through a vector.  We then discuss a model for disease transmission in a population of vectors interacting with a population of hosts.  We examine the effect of different control strategies, including  preventive measures as well as  treatment, and also possible effects of changes in the environment.     We point out some of the complexities inherent  in the biological processes, and the variability resulting from change in the values of the parameters in the system.