The application of system theory (or more precisely, differential equations) to immunology and disease, in general, is presented here. Particular results from U.S.-Russian research collaboration depict the potential role of such systematic analysis for more effective health care and disease control. In particular, some emphasis is given to control of influenza. After a brief systematic overview of immunology, a simple infectious disease model is developed to explain four basic forms of disease: subclinical, acute, lethal and chronic. Then, disease treatment is studied.