As serious as are the consequences of invasive species that directly cause human afflictions through their production of lethal protease inhibitors (Bryonia alba), allergens (Parthenium hysterophorus) or furanocoumarins (Hercaleum mantegazzianum), other introduced species may cause even greater risks to human health by enhancing the proliferation of vectors of virulent human parasites. The dense, floating mats of Eichhornia crassipes (water hyacinth) create habitat for larvae of the dipteran vectors of Plasmodium spp., the causative agents of malaria, and other parasites. Facilitation of a human parasite is not restricted to aquatic systems. In Africa, the tropical American shrub Lantana camara (lantana) provides essential habitat for dipteran vectors (Glossina spp.) of protozoans (Trypanosoma spp.) that cause trypanosomiasis. Unanticipated health consequences will likely continue to emerge from new plant introductions. Sin Nombre Virus (SNV) is a rodent-borne parasite that causes Hantavirus Pulmonary Syndrome, an often-lethal disease in humans. Populations of rodent vectors of SNV in South America increase rapidly in response to synchronous fruit availability among masting, native bamboos. With depletion of this temporary food source, the rodents seek food near human settlements, increasing the risk of human infections with SNV. In the United States the omnivorous deer mouse Peromyscus maniculatus is also a SNV carrier. The escape of Asian cold-tolerant bamboos from cultivation raises the possibility of invasions (several have already become naturalized) and providing a temporary boost to populations of infected native rodents. Proposed introductions of aquatic vascular species, species with masting reproduction and those that would occupy an unfilled niche in the proposed new range deserve careful evaluation for their possible roles as unforeseen catalysts of species interactions, especially of human parasites.