Acquired immunity emerged sometime at the beginning of vertebrate evolution as defense mechanism against the infectious diseases caused by pathogens. During this evolution the amazing mechanism including reassembly of genetic segments was acquired probably by fortuitous events. We encountered the molecule called Program cell death 1 (PD-1) in 1992 and then revealed PD-1 is the major braking system in acquired immunity. In 2002, we discovered PD-1 blockade can cure tumors in mouse models by reactivating the acquired immunity. In 2014, 22 years after PD-1 discovery, the treatment of cancer by PD-1 blockade is approved by PMDA and FDA, and considered to be revolutionary cancer treatment which is often compared to the penicillin discovery for the treatment of infectious diseases. The success of cancer treatment by PD-1 blockade owes to acquired immunity. During the long history of fight against diseases, we almost succeeded to conquer infectious diseases in the last century. Fortunately, we may be able to overcome cancer the last major life-threatening disease, by the same mechanism applied to infectious diseases.