A three-year-old boy has been left partially paralysed after a virus which presents itself like a common cold triggered a rare “polio-like” disease.

Jacob and his family, from the NSW Central Coast, had been enjoying a day out in Sydney on April 14 when he complained of feeling achy and had a bit of a temperature, his mother Sheryn James told nine.com.au.

Having just had a cold herself, Mrs James said she assumed Jacob was coming down with the same thing and gave him some Panadol.

“He picked straight back up, so we didn’t think anything else of it,” she said.

Jacob James is being treated in Westmead Children's Hospital's ICU. (Facebook)

The next morning Jacob also seemed happy and well, Mrs James said.

“It was Jacob’s dad’s birthday and he got out of bed, really excited because he was giving his dad a big Nerf gun for his birthday.”

“He was running around the house shooting his dad with Nerf bullets.”

It was then that his parents noticed something unusual about Jacob’s left arm. It was hanging limply by his side.

“We said, ‘Why aren’t you moving your left arm?’ We picked his arm up, and we kept saying, ‘Keep your arm up, Jacob,’ and it would just fall down.”

“Jacob was laughing when I was picking his arm up. I said, ‘Why are you doing that? He said, ‘I can’t leave it up, I can’t move it.’”

Worried and confused, Jacob’s parents rushed him to the emergency department at Gosford Hospital.

“They were absolutely amazing there. We went straight to triage and then right away they knew this was not right,” Mrs James said.

Jacob James and his mother Sheryn share a cuddle in hospital. (Facebook)

An MRI showed that part of Jacob’s spine was swollen and he was quickly transferred by ambulance to the Westmead Children’s Hospital in Sydney.

Over the coming days, doctors struggled to find an answer to what was causing the swelling on Jacob’s spine and his condition began to deteriorate.

“On the Wednesday, he started to struggle to breathe, so they put him on some oxygen. And then on Friday night he just went downhill so fast,” Mrs James said.

“You could see as every hour went by from midnight to 6am he just deteriorated right in front of our eyes.”

Jacob was rushed to ICU where he was intubated.

“Afterwards they told us if we didn’t intubate him when they did he would have died within the hour.

“And at that point we still didn’t know what was wrong. They did all these tests on him and everything was coming back negative.”

Jabob James has been busy winning over the hospital's nurses and doctors with his trademark grin. (Facebook)

After three weeks, a test finally came back positive, for the common enterovirus.

Although the virus usually only causes mild symptoms, doctors believe the virus then triggered a rare, polio-like disease – acute flaccid myelitis.

Polio, a disease that crippled thousands of children around the world before a vaccine was created, is itself a strain of enterovirus.

Mrs James said Jacob’s doctors had remarked on the similarities of his symptoms to polio even before they knew the cause.

“Straight away, as soon as we got to Gosford, everybody was saying this looks like polio, and they said that at Westmead too. They said we can safely say it’s not polio, but it’s presenting like polio.”

Acute flaccid myelitis, which is sometimes referred to as the “modern-day polio”, is thought to affect just one to two people in one million per year, and is mainly seen in children.

In the US, health officials last year expressed concern about a possible rise in cases of the rare disease, which can cause paralysis in the arms and legs.

Jacob is still unable to move his left arm. His diaphragm is also paralysed and he needs a ventilator to help him breathe.

Doctors expect he will remain in the ICU for the next six months.

Mrs James said her son had been coping remarkably well with his sudden illness.

“He is amazing. He just blows our mind in how mentally strong he is. He is still so happy, so playful,” she said.

“The day he was intubated, that afternoon he still opened his eyes and smiled at me and said hi.

“Everyone here is just amazing with him. They get him out of his room and we go out to the park every single day to play and get some sun.

“He has a little ride-on electric car that he sits on and they let him drive around the ward in his car.”

After being unable to talk for several weeks because of a tracheotomy, Jacob had his first conversation with his father over the phone last night, Mrs James said.

“He called dad up last night … and he said, ‘miss you’, and it came across so loud and clear and then he said ‘I love you’.

“It was just absolutely beautiful.”

Always positive, Jacob had been coping remarkably well, his mother said. (Facebook)

Doctors at Westmead Children's Hospital were now consulting with medics in Cleveland about the possibility of fitting Jacob with a diaphragmatic pacemaker, Mrs James said.

With Mrs James unable to work her full-time job, and her husband needing to cut back to part-time hours, an online fundraiser has been set up to help the relieve the family’s financial stress.

The family has also started a Facebook page to share Jacob’s story and raise awareness about acute flaccid myelitis.