The Horsley parents remain by their two-year-old daughter's side at Sydney Children's Hospital, as she battles to recover from Guillain-Barre Syndrome.

The rapid onset autoimmune disorder causes the body to attack its own nerves, causing various levels of weakness and paralysis. Most people recover, though some suffer lingering symptoms and in some cases it can be fatal.

The cause of the condition is unknown, but it's often triggered by an infection like the flu. But for Ava, and her parents, there was little warning of the medical emergency that would arise a fortnight ago.

"On the Friday (March 22) Ava was fine, and went to bed as normal," Mrs Northey said, "but the next morning she woke up vomiting and had a funny walk.

"She just seemed to be getting worse so we called Triple Zero, and the paramedics transferred her immediately to Wollongong Hospital.

"She underwent a number of tests which didn't provide us with any answers and then she started to have absent seizures - where she'd just blank out for a while."

Ava was transferred to the Randwick hospital on Monday, March 25, where she was seen by neurologist Dr Hugo Sampaio, who made the diagnosis.

"Guillain-Barre is a very rare condition that affects around one in 70,000 people worldwide," Mrs Northey said. "It often starts as weakness in the feet and legs and works its way up a person's body, and can lead to difficulty with talking, swallowing or breathing.

"Fortunately for Ava it has stopped at her trunk. She has lost her ability to walk, but she's getting there slowly and we hope she'll make a full recovery in the next few weeks."

There's no known cure for Guillain-Barre syndrome, but several treatments can ease symptoms and reduce the duration of the illness. 

Ava has undergone intravenous immunoglobulin therapy, which uses antibodies from blood donors to alter the abnormal immune response.

According to the Red Cross Blood Service, it takes around 10 donors to make each treatment of IV immunoglobin - which is used for a range of autoimmune illnesses.

"It if wasn't for blood donors, Ava wouldn't be recovering," Mrs Northey said.  "We're also grateful for the hospital staff, and for Ronald McDonald House which has provided accommodation so we can be near Ava.

"From being so helpless when she went downhill, to now see her recovering is such a relief.

"We just really wanted to let other parents know how important it is to seek medical help early when they think something isn't right and to keep pushing for answers."