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."