Alvarez, 37, was first referred to the Barrow Neurological Institute at St. Joseph's Hospital and Medical Center in Phoenix with balance problems, difficulty swallowing and numbness in her left arm.
An MRI scan revealed a foreign growth at her brain stem that looked just like a brain tumor to Dr. Peter Nakaji, a neurosurgeon at the Barrow Neurological Institute.
"Ones like this that are down in the brain stem are hard to pick out," said Nakaji. "And she was deteriorating rather quickly, so she needed it out."
Yet at a key moment during the operation to remove the fingernail-sized tumor, Nakaji, instead, found a parasite living in her brain, a tapeworm called Taenia solium, to be precise.
"I was actually quite pleased," said Nakaji. "As neurosurgeons, we see a lot of bad things and have to deliver a lot of bad news."
When Alvarez awoke, she heard the good news that she was tumor-free and she would make a full recovery. But she also heard the disturbing news of how the worm got there in the first place.
Nakaji said someone, somewhere, had served her food that was tainted with the feces of a person infected with the pork tapeworm parasite.
"It wasn't that she had poor hygiene, she was just a victim," said Nakaji.
"We've got a lot more of cases of this in the United States now," said Raymond Kuhn, professor of biology and an expert on parasites at Wake Forest University in Winston-Salem, N.C. "Upwards of 20 percent of neurology offices in California have seen it."
The pork tapeworm has plagued people for thousands of years. The parasite, known as cysticercosis, lives in pork tissue, and is likely the reason why Jewish and Muslim dietary laws ban pork.
Kuhn said whether you get a tapeworm in the intestine, or a worm burrowing into your brain can depend on how you consumed the parasite.
Eat the parasite in tainted meat and you'll end up eating the larvae, called cysts. Kuhn said in that case, a person can only end up with a tapeworm.
"You can eat cysts all day long and it won't get into your brain," said Kuhn. Instead, the larvae go through the stomach and mature in the intestine.
"When it gets down into their small intestine, it latches on, and then it starts growing like an alien," said Kuhn.
Once there, the tapeworm starts feeding and gets to work. A single tapeworm will release 50,000 eggs a day, most of which usually end up in the toilet.
"They can see these little packets pass in their feces," said Kuhn. "And ... sometimes people eat the eggs from feces by accident."
Kuhn said it is then feces-tainted food, and not undercooked pork, that leads to worms burrowing into the brain.
Unlike the cysts, the eggs are able to pass from the stomach into the bloodstream. From there, the eggs may travel and lodge in various parts of the body -- including the muscle, the brain or under the skin -- before maturing into cysts themselves.
According to Kuhn, who has traveled to study this parasite, cysticercosis is a big problem in some parts of Latin America and Mexico where health codes are hard to enforce and people may frequently eat undercooked pork.
As people travel across the border with Mexico for vacation and work, Kuhn said so does the tapeworm. One person infected with a parasite, who also has bad hand washing habits, can infect many others with eggs.
"These eggs can live for three months in formaldehyde," said Kuhn. "You got to think, sometimes, a person is slapping lettuce on your sandwich with a few extra add-ons there."
Dr. Christopher Madden, an assistant professor in the University of Texas Southwestern department of neurological surgery in Dallas, has operated on a number of these cysts himself. He said not every worm needs to be surgically removed; those whose location is not an immediate threat to the patient's health can be treated with medications that cause the worms to die.
But when the cysts are in problematic locations, as was the case for Alvarez, an operation is necessary. Fortunately, the long-term prognosis for most patients is positive.
"Most patients we see actually do very well with medicines and/or surgery to take out a large cyst," Madden said.
Alvarez is not alone in accidentally eating tainted food, but Nakaji rarely sees cases so severe that people require surgery. Nakaji said he only removed six or seven worms in neurosurgery this year.
"But lodging in the brain stem is bad luck," he said.
Nakaji said other parts of the brain have more "room" or tissue to expand around a growing cyst. However the brain stem, which is crucial to life, is only the width of a finger or two.
"She could have recovered," said Nakaji. "But if the compression lasted for long enough, she could have been left permanently disabled or dead."