This paper deals with the history of “pathogenic environments,” understood as places, regions, or environments whose characteristics are considered to be the origin of diseases in the human beings. While some specific environments were almost universally considered noxious, some others had a different trajectory. Crowded and poorly-ventilated premises as well as tropical regions were perceived as “the most unhealthy spots in the world.” However, the progressive “medicalisation” of hospitals transformed what were previously considered to be hellholes into therapeutic places. This does not mean that iatrogenic diseases disappeared, but that hospitals tended to be seen in a more positive way. Similarly, European colonial expansion changed medical perspectives on tropical regions. Western physicians became increasingly convinced not only that could they prepare Europeans for long travel, but also that they could shape foreign environments. For instance, the perception of the Caribbean climate gradually changed “from deadly to healthy” from the middle of the 19th century onwards. Changes in perceptions could thus follow scientific progress, but also arise from political agendas and stigmatising narratives. Victims of the harmful influences of a pathogenic environment were often presented as responsible for their fate. For instance, some were accused of not carrying out the instructions delivered by the administrative or medical authorities. The process of (un)making pathogenic environments thus offers privileged access to the understanding of the relationship between health and environment. This paper provides situated accounts of how some places or regions came to be perceived as pathogenic or ceased to be so. It explores avenues of research, such as the study of lay perspectives on health and environment, the dangers of travel itself, and the connection between environmental health and military medicine.