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A New York City firefighter and his wife lost their 4-month-old daughter to the coronavirus, the family said.

Jay-Natalie La Santa died on Monday, nearly a month after she was rushed to the hospital with a fever.

"She was very feisty. She was a little Angel with the most beautiful smile. She kept us on our toes," the girl's grandmother, Wanda La Santa, told NBC News in a phone interview on Thursday.

The baby, born in November, was the first child for Jerel La Santa, a firefighter, and his wife, Lindsey La Santa. She was Wanda La Santa's first grandchild.

Jerel La Santa holds his daughter, five-month-old Jay-Natalie. Courtesy of Wanda La Santa

"Jay-Natalie had everybody wrapped around her finger. Whatever Jay-Natalie wanted, she got," Wanda La Santa said.

The baby was rushed to the hospital on March 21 after she developed a fever. Wanda La Santa said Jay-Natalie was born with a heart condition and she was admitted right away. Her initial test for the virus came back negative.

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Tests for Wanda La Santa and the baby's parents also came back negative. After nearly a week in the hospital with no improvement, Jay-Natalie was tested again for the coronavirus and this time it came back positive.

"We have no clue how she got it," Wanda La Santa said.

Wanda La Santa holds her granddaughter, Jay-Natalie. Courtesy of Wanda La Santa

The first 16 days Jay-Natalie was in the hospital "she was awake, she was fighting," her grandmother said. Doctors eventually placed the child in a medically-induced coma because of her heart condition.

After about a week in the coma, she started to show signs of improvement.

"She had broken the fever. It looked like it was subsiding," Wanda La Santa said. "Her lungs were starting to clear."

The family had a meeting with doctors on April 20 and everyone believed Jay-Natalie was on the mend and things were moving in the right direction, but just hours later "everything just started going south," Wanda La Santa said.

"In a matter of minutes, her pulse started to drop, her breathing started to drop and she went into cardiac arrest," the grandmother said.

Jay-Natalie died at the hospital on April 20, just one week shy of turning 5-months-old.

Wanda La Santa says the family is devastated over the sudden loss. She says she wanted to share the tragedy her family is going through to encourage other people to take the virus seriously.

"I want people to understand and know this is something serious. This is not a joke," she said. "My granddaughter fought a big battle for a whole month in the hospital. She gave it her all."

The New York City Fire Department said Jerel La Santa has been a firefighter since November. "Our thoughts and prayers are with the La Santa family during this tragic time," the agency said in a statement.

The FDNY Hispanic Society asked people to keep the family in their prayers.

"Rest in peace little one, God bless you," the fraternal organization wrote in an Instagram post.