While considerable uncertainty remains about the total economic costs of invasions, estimates of the economic costs of particular invasives to particular sectors indicate the seriousness of the problem. Many of these estimates remain controversial among economists. Even worse, the global changes that are affecting many parts of the world are expected to lead to the expansion of the ranges of many viruses that are potentially dangerous to humans. When people move into formerly unoccupied wilderness areas, this brings them into contact with a wider range of viruses and bacteria, while air travel carries them around the globe before the symptoms become apparent. Unfamiliar types of infectious agents, either acquired by humans from domesticated or other animals, or imported inadvertently by travellers, can have devastating impacts on human populations.
Pathogens can also undermine local food and livestock production, thereby causing hunger and famine.
The viruses carrying smallpox and measles spread from Europe into the western hemisphere shortly following European colonization. The low resistance of the indigenous peoples to these diseases helped bring down the mighty Aztec and Inca empires. These strains can then infect the pig-tending humans, who then infect other humans, especially through rapid air transport. At least some invasive species of plants may themselves be considered as a health hazard in both temperate and tropical regions. In the close vicinity of habitations, both Schinus terebinthifolius and Melaleuca quinquenervia appear to cause respiratory difficulties in many people, and skin contact with leaves and the sap of S. terebinthifolius results in red, itching rashes.