While the world is focused on the fate of hundreds of girls in Nigeria who have been kidnapped in recent months by the extremist Islamic group, Boko Haram, it might be a good time, activists say, to talk about how the trafficking of humans is a global problem.

“What’s happening in Nigeria is just the tip of the iceberg,” said Terry FitzPatrick—the communications director for Free the Slaves, a non-profit organization in Washington, D.C., dedicated to ending slavery.

“It’s important to think about rescuing all the children who are enslaved throughout the world,” FitzPatrick told Fox News Latino.

The United Nations’ International Labour Organization estimates that 21 million people are being held in some form of slavery or another, but other groups put the figure closer to 30 million. More than one-quarter of them are children.

“The misperception is that sex slavery is the biggest problem,” FitzPatrick said, “but only 22 percent of people in slavery are in the sex trade.”

The conditions that give rise to human-trafficking—poverty, the lack of education, large numbers of people marginalized for reasons of class or ethnicity or race—are often present in places like Mexico, where large numbers of people are trying to cross the border.

“Millions of people are on the move around the world,” FitzPatrick observed. “Most of those are looking for work. Places like the U.S.-Mexico border, that’s a Wild West environment where human traffickers can pose as legitimate labor recruiters.”

FitzPatrick estimated that there are 60,000 people in the United States who have been tricked or coerced into doing work that they cannot leave. And even if they aren’t in the sex trade, he said, “From interviewing women coming out of that sort of situation, it’s our understanding that most have experienced some element of sexual exploitation.”

A memorable New York Times magazine article from 2002 described women and girls from Eastern Europe, Asia and Africa being smuggled into the U.S. through the border with Mexico.

According to the National Human Trafficking Resource Center, California and Texas rank No. 1 and 2 in suspected instances of trafficking, together accounting for nearly 60 percent of all cases.

In a high-profile case, Charles Marquez, an El Paso, Texas, man was convicted of seven counts of sex trafficking last year. For five years, Marquez posted ads in Ciudad Juarez newspapers for fake jobs in order to attract his victims.

"The El Paso area is an intense place for trafficking, both forced labor and sex trafficking," John Martin, director of the Center of Hope, told the El Paso Times last year.

But those are by no means the only place those sorts of things happen. A Baltimore-area businessman named Alarcon Wiggins pleaded guilty for taking part in a sex trafficking ring that promised young women careers in the music industry.

A 2014 study of the commercial sex industry by the Urban Institute found that pimps make between $5,000 and $33,000 a week. The underground sex trade in Atlanta was estimated to be worth $290 million in 2007, "nearly 2.5 times bigger than the 2013 payroll of the Atlanta Falcons."

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Bill Vourvoulias (@bvourvoulias) is an editor at Fox News Latino.

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