Story highlights An allergic reaction to an antibiotic sent Yaasmeen Castanada to a hospital burn unit

"It has just spiraled into a nightmare," her aunt told CNN affiliate KABC

A doctor says the teen's prognosis is good, but she's in critical condition

(CNN) It started with a sore throat on Thanksgiving and an antibiotic from a friend who wanted to help.

Now 19-year-old Yaasmeen Castanada is fighting for her life inside a California hospital's burn unit, suffering from an allergic reaction that's so severe she has large open wounds all over her body.

"It is heartbreaking, every day is a different look. Every day, she's like, shedding away. ... Overnight, it's a whole different person that you're looking at," Martha Hughes, Castanada's aunt, told CNN affiliate KABC

Doctors diagnosed Castanada with Stevens-Johnson Syndrome, a rare disease that can be triggered by antibiotics or other medications.

"When she took the medication, she started having a hard time breathing, and she told her mom that her lips were burning, her throat, her eyes, they got so red that she couldn't talk. So she rushed her to the ER, and that's when they diagnosed her with the disease. And from there it has just spiraled to a nightmare," Hughes said.

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