Robert Koch is the pioneer of the new field for pathogenic bacteriology and promoted the causal therapy in infectious diseases, succeeded by the two streams to serotherapy and chemotherapy. Shibasaburo Kitasato, a disciple of R. Koch, developed master’s thoughts and established the serotherapy using antitoxin against tetanus. P. Ehrlich, another Koch disciple, discovered salvarsan in cooperation with Sahachiro Hata and developed the chemotherapy. The discovery of penicillin by A. Fleming and its “rediscovery” by H. Florey, E. Chain and others led to rapid advances in chemotherapy for the treatment of infection. After then Japanese researchers have greatly contributed on the research and development of many antimicrobial and antiparasite agents. Nevertheless, the emergence of superbugs, such as MRSA, PRSP, VRE and MDRP has become threat in bacterial infections, requiring not only novel antimicrobials but also the innovated approaches to overcome the limitation of antimicrobials. On the other hand, the disruption of the wilderness, global warming and the world-wide rapid transportation have caused the emergence of new infections, such as HIV, SARS, tuberculosis and bird flu. Recent advancement of biotechnology has enabled the serotherapy more practical and for example humanized or fully human antibodies have just come into use.