As the United States makes it more difficult to cross its borders illegally, business for human traffickers has been booming with smugglers making billions of dollars a year; traffickers make an estimated $6.6 billion each year by bringing immigrants across the U.S.-Mexico border; in May, x-ray machines at checkpoints in southern Mexico found 513 people crammed into two trailers, leading to the largest bust yet; the United States has inadvertently generated a boom in the smuggling business by making the border harder to cross; drug cartels are believed to have entered the lucrative market