By late August, the epidemic had swiftly spread over most of northern and western Europe, including all of England and Wales, as well as eastern Ireland. Its early spread was connected to the rise of an international seed potato trade, but once established, the blight expanded exceptionally quickly in the especially wet summer of 1845. Within a few months, it was reported in most European countries north of Spain and south of Russia. The disease followed the agricultural practices of its time: farmers grew many varieties of potatoes, some resistant to pests and others not, and they used common cultivation methods that moved across national borders. Without a single act of vandalism or theft, the blight had become globalized. The outbreak proved so devastating that it left millions of people without food for years. In fact, the famine that ensued in Ireland is estimated to have killed between 810,000 and 1 million people, or about 10 percent of the population. Modern scientists believe that the blight originated in South America and was spread by travelers on ships carrying seeds around the world. In conclusion, the Irish potato famine was caused by a combination of factors, but the spread of the disease itself was due to the way that modern agriculture works. Seeds from different varieties or species of potato were mixed together during planting, so if one variety was more susceptible to the pest than another, then everyone else would be too.