An unintended, but destructive consequence of European arrival was the introduction of foreign diseases. It is believed that a Hudson's Bay Company trapping expedition inadvertently introduced malaria into the Central Valley in 1832. Over the next few years successive waves of malaria swept this region, killing thousands of Native Americans in the Central Valley who had no natural immunity to European diseases. The spread of disease was exacerbated by both the massive numbers of mosquitoes in the Central Valley wetlands, and the densely packed Native American villages. As many as 80 percent of the Plains Miwok who had survived Spanish colonization died in the consequent epidemic. A smallpox epidemic struck around 1837, claiming even more lives, including that of Estanislao, who died at Mission San Jose in June 1838.