Diseases and/or medical conditions often involve more than one system. in fact, the effects on a secondary system may be as serious as the primary system most associated with the problem. for example, heart disease can often cause fluid build up in the lungs. the primary system in this example is the cardiovascular system; the secondary system is the respiratory system. fluid in the lungs will prevent the lungs from functioning properly causing the respiratory system to fail, yet the problem started with the cardiovascular system. describe another medical condition or disease where the primary system causes a serious problem with a secondary system.