A 4-year-old California girl is fighting for her life in a pediatric intensive care unit after ingesting a button battery.

When Camila Romero was admitted to Loma Linda University Children's Hospital in Loma Linda, California, on March 8, her parents said she was struggling with flu-like symptoms, including a cough, fever, fatigue, and loss of appetite.

"[Camila] kept saying, 'Mommy, I'm hot,' and I said, 'OK, baby, we need to take your medicine.' And she was like, 'No,'" mom Cassandra Tafolla told ABC News. "She was telling me her throat burned. She was telling me that she couldn't breathe."

Dr. Harman Chawla, medical director of the pediatric intensive care unit at Loma Linda University Children's Hospital and one of Camila's doctors, said the 4-year-old was being treated for virus symptoms until doctors ordered an X-ray for her and discovered she had somehow swallowed a button battery.

"One of the things that really struck me about her story, and which made me a little worried as a parent, was the fact that she didn't come in for this ingestion. She came in because her lungs had a virus. That's how quick it is," Chawla told ABC News. "These kids aren't sucking on the button batteries. It's in the mouth and it's swallowed. And that's unfortunately what winds up happening."

For Camila's parents -- Tafolla, 24, and dad Hugo Romero, 33 -- the button battery discovery came as a shock.

"I heard of [button batteries, but] I just never had anything that will use that type of battery," Romero said.

Tafolla said she "didn't know what a button battery was, honestly," adding that she wasn't aware of any items in her home that used button batteries.

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Over 3,500 people in the U.S., both kids and adults, swallow button batteries each year, according to the nonprofit National Capital Poison Center. These small batteries -- 20-millimeter diameter lithium coin cells are one example -- show up in many common items, including audio greeting cards, jewelry, remote controls and toys.

The injuries they cause, if ingested, can be severe and even fatal.

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Chawla said they can also cause ongoing injuries over time.

"[A button battery] changes the environment of the mucosa of the esophagus. And so, it can really be one of those things that is progressive and actually damages the surrounding structures, including blood vessels as well," he said.

According to Chawla, Camila underwent surgery to remove the button battery, but she remains sedated and in critical condition in the pediatric intensive care unit, because she's still battling a respiratory virus, and the battery eroded her airway, causing inflammation and damage.

"Her prognosis is guarded," Chawla said. "She's still got a lot of healing to do … We're optimistic that we're going to be able to get her through this, but … a lot of things have to go the right way, with the right timing, for her to get that outcome."

Tafolla said as Camila remains in the hospital, one of her relatives has started an online fundraiser to support her medical expenses and the family's living expenses.

Tafolla and Romero said they want to raise awareness about how dangerous button batteries can be for kids.

"Please double check your children's toys," Tafolla said. "Just make sure it's very well secured before opening and giving it to our children that are going to be playing with it."