Alexandre Yersin, the great French discoverer of yersinia pestis, was a keen explorer of unknown lands. At the age of 30, a member of the French Colonial Health service, he set off to fulfil his intimate dream and explore other continents. For almost two years and three long expeditions, he journeyed through widely unknown regions in the province of the French Indochina, in southeast Asia, territories of Vietnam, Cambodia and Laos. This article presents vignettes from his explorations. During his difficult travels, he carefully planned and noted his itineraries; designed new routes, but also observed and recorded sociodemographic and environmental data and unidentified diseases. The immature science of late 19th century geography had the strength to allure such an influential medical figure and place him among the early medical geographers. His journeys, observations and recordings brought to Yersin great experience, and he made his most important scientific contributions after he had concluded his explorations.