Nine-year-old Yrene returned from school one day to her rural home in central Mexico to learn that her father had died of a heart attack.

As a legal U.S. resident, the family's only breadwinner had split his time between Mexico and the United States, working in the fields to keep his wife and nine children from drowning in poverty. But now he was gone.

Yrene's mother, who did not have a permit to work in the United States, was desperate to provide for her family, so she took a risk. She drove to the border and told U.S. customs agents that she and her children were tourists. They got in.

Two decades later, Yrene is a single mother with three girls, ages 6 to 12. She's spent the last nine years sorting and labeling peaches, plums and apricots at a Tulare County packinghouse, working her way up to supervisor.

Yrene is a prime example of how the Valley's don't-ask, don't-tell policy allows farmers to keep their businesses humming. Yrene's employer knows she's here illegally, but he says his company depends on hard-working people like her to get the job done. Yrene, in turn, depends on her job to pay the medical bills for her eldest daughter, who was born with one kidney and has scoliosis, a curvature of the spine. She doesn't receive government aid.

"We know it's not legal," Yrene said. "I would like to stay in my home country and get a job and a better life, but really there is no way to live there."

When Yrene crossed the border in 1990, the United States had an estimated 3.5 million illegal immigrants. But thanks to lax border control, inconsistent employer enforcement policies and a visa system that allows only a trickle of immigrants each year to come here to work, that number has soared.

Immigrants — mostly Mexicans — have duped customs agents with fake passports, overstayed their visas, sneaked across the border hidden in cars and trucks, trekked through the scorching Arizona desert or waded across the Rio Grande.

Until recently, more than half a million immigrants were entering the United States illegally each year — about 50,000 to 100,000 of them pouring into California, according to the Public Policy Institute of California.

Today, Yrene is one of some 11 million illegal immigrants in the United States. More than 2.5 million of them are in California, according to estimates by the Pew Hispanic Center.

The San Joaquin Valley is an especially popular destination because of its low cost of living and abundant agriculture jobs, experts say. One estimate by the Urban Institute found that more than 200,000 illegal immigrants were in the Valley in 2004 — although a large number of seasonal workers weren't counted. The vast majority were from Mexico.

Most illegal immigrants say they came for the money — to better provide for their families. The average 25-year-old Mexican man makes about 3 1/2 times more here than in his home country — even taking into account the higher cost of living in the United States, said Gordon Hanson, an economics professor at the University of California at San Diego.

Still, the jobs they take are almost always low-wage. They make up more than a quarter of grounds-keepers, housecleaners, construction workers and dishwashers in the United States, the Pew Hispanic Center found.

Half to two-thirds of farmworkers are illegal immigrants, according to several experts. The share is likely higher in California, where a U.S. Department of Labor survey in 2006 found that 98% of farmworkers are immigrants — legal and illegal — compared to 75% nationally. Some agriculture industry leaders say up to 90% of farmworkers in the Valley are illegal immigrants.