Circulatory disease is the number two cause of death next to cancer in Korea, while cardiovascular disease alone is the number one cause of death in the US. Among circulatory diseases, the most intriguing feature of atherosclerosis is that it occurs prevalently at the sites where the endothelium cells are deformed by low wall shear stress (WSS), causing damage to the tunica intimae. Thus, it is shown how computational fluid dynamics (CFD) and medical imaging technique (MIT) can contribute to the diagnosis and the treatment of circulatory diseases in such atherosclerosis-prone sites of blood vessels. On the other hand, people have still suffered from many diseases caused by abnormal proliferation of cancerous cells or bacteria. Thus, it is discussed how human tissue cells and bacteria react to their external fluid media and how their motilities can be analyzed quantitatively by using micro-optical technique and microfluidics.