In ancient China, Japan, and Korea it is clear that roundworm and whipworm were the most common parasites present. Where data are available to estimate how common these parasites were, they suggest that more than half of the population were infected. Flukes spread by eating raw fish and crustaceans were a significant health risk, but less than half of individuals appear to have been affected. These include Chinese liver fluke, Fasciolopsis, Gymnophalloides, Metagonimus, and Paragonimus. The range of species of flukes present seem to have been higher in Korea and Japan than China, which may reflect the range of species endemic there, or the range of foods traditionally eaten raw in each culture. Oriental schistosomiasis is contracted by wading in still freshwater, so farmers growing rice in paddy fields were at particular risk. Major trade routes such as the Silk Road have been shown to act as conduits for the spread of intestinal parasites across East Asia.