From NPR:
The vast majority of that estimated 11 million come from Mexico. Others come from Central America and countries like China, the Philippines and India. Some enter the U.S. illegally, but there are many others who come to America legally, says Ruben Rumbaut, a sociology professor at the University of California, Irvine, who focuses on immigration. "Perhaps 40 percent of the 11 million or so are here because they overstayed their visas," Rumbaut says. For some parts of the country, the undocumented in America may seem like a distant population, Rumbaut says "We are a country of over 300 million people, and so 11 million might not seem that large a share," he says. "But you would be surprised how many people are tied in one way or another to someone who came to the United States on undocumented status."

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