When Japan was opened to the outside world during the last days of the Tokugawa shogunate, the ocean made trade possible with East Asia and the Western Powers but at the same time created an environment in which highly contagious diseases could enter the country. The subjection of foreign registered ships to quarantine was more than just a routine port operation, however, as it also signaled Japan's arrival as a modern state. This article introduces past and current conditions in Japan's treaty-ports quarantine operations.