The Pennsylvania girl, Sarah Murnaghan, who received a second lung transplant, has developed pneumonia, her mother shared on Facebook.

Doctors believe it is caused by "aspirations from her belly," Janet Murnaghan wrote in a Facebook post Monday. "Yesterday was tough. Today she is more stable, but this is definitely a large setback." Sarah Murnaghan, 10, has aspiration pneumonia in her right lung which is an inflammation of the lungs or airways which can lead to the lungs from breathing in foreign material, such as food and saliva. Antibiotics may be used to treat the condition.

In the same post, Janet Murnaghan thanks everyone for their thoughts and prayers.

"We have an amazing team of doctors who go above and beyond but also walk this road with us in such a kind and compassionate way," she wrote. "Thankful to God for this day!"

Murnaghan's parents successfully fought a rule preventing her from qualifying for adult lungs underwent two lung transplants in June. The first transplant failed almost immediately after surgery June 12. Murnaghan was then put on the waiting list for another adult-lung transplant which she underwent surgery in June 15, even though the donor lungs were infected with pneumonia. However, doctors say the pneumonia Sarah has is a new infection.

Pneumonia is not uncommon after any type of lung surgery, Dr Sanjay Gupta said. "Sarah had an even higher increased risk because she's taking immune-suppressing drugs and because of her cystic fibrosis," Gupta said, according to CNN. "But even with a setback like this infection, nearly 80% of people survive at least one year after lung transplant."

The second transplant appeared to be an initial success after she took a few breaths on her own, but she was put back on the ventilator because of partial paralysis of her diaphragm, a complication from the last transplant. Sarah had diaphragm plication to treat the paralysis on July 2 at Children's Hospital of Philadelphia. The surgical procedure involves the folding of the diaphragm, allowing more volume inside the chest cavity for the lungs to accept air.