A Sydney baby has become seriously ill after contracting measles two months before he was due to be vaccinated.

Key points: Liam Eldridge visited the Philippines and contracted measles shortly before he was due to be immunised

Liam Eldridge visited the Philippines and contracted measles shortly before he was due to be immunised His parents say it was "terrifying" seeing their young boy helpless and ill

His parents say it was "terrifying" seeing their young boy helpless and ill Experts have warned Australia cannot afford to let immunisation rates fall as measles remains a major problem in other countries

Eleven-month-old Liam Eldridge developed a high fever and a rash after visiting family in the Philippines last month.

An outbreak in that country has so far killed more than 315 people this year.

At first, Liam was misdiagnosed with the mild viral infection Roseola — a condition that rarely requires medical intervention.

Measles cases occur in Australia when it is brought in from overseas. ( Supplied: Centers for Disease Control and Prevention )

"It was just terrifying for us as new parents," said Liam's mother Shendie Requillo.

"No-one would want that or wish that on their child."

Liam's father, David Eldridge, said he's since learned babies as young as nine months can be given the MMR (Measles, Mumps, Rubella) vaccine, something he would have done if the family was aware of the infection rates in the Philippines.

"It was very scary, not being able to do anything for your little, helpless baby," he said.

"We had to just sit there and watch and hold him down if they needed to take blood."

Liam Eldridge contracted measles in February when he was 10 months old. ( Supplied )

Virus sparks major concern overseas

Liam's parents said that fortunately, the boy recovered and was thriving.

But around the world, measles rates doubled last year and advanced economies are struggling to contain the disease.

In the United States this week, the New York County of Rockland declared a state of emergency and banned unvaccinated children from public places for 30 days while the community grapples with the longest measles outbreak for 20 years.

While the virus has been officially eliminated in Australia, 77 cases have been recorded in 2019.

Vicky Sheppeard, the NSW Health Director for Communicable Diseases, said measles cases do not occur in Australia unless it is introduced from overseas.

"[Cases occur] particularly when it's in our near neighbours like the Philippines and Thailand," Dr Sheppeard said.

"With the frequency of travel between those countries and Australia, we inevitably get those cases here."

Immunisation of both children and adults, who may need a booster shot, remains the best protection.

"A lot of adults in their 20s, 30s and 40s unknowingly missed out," Dr Sheppeard said, adding that the vaccine is free for people born after 1966.

Shendie Requillo said it was "terrifying" to see her son Liam ill. ( ABC News: Nicole Chettle )

Experts issue warning

Kristine Macartney, the Director of the National Centre for Immunisation Research, said Australia cannot afford to be complacent with a virus that's more infectious than influenza and tuberculosis and can have rare, fatal consequences.

"Children can be well for a number of years and then they'll develop a severe neurologic condition leading to death," she said.

Professor Kristine Macartney says Australia cannot afford to be complacent. ( ABC News: Nicole Chettle )

"In my early career, I looked after children with subacute sclerosing panencephalitis and they died. There was nothing we could do. It was heartbreaking."

Professor Macartney issued a warning to all Australians.

"If measles takes off and thousands of people are infected like we're seeing in other parts of the world, then some of those children will develop this complication years later," she said.

"If we take our guard down, if we let immunisation rates fall, we will see measles. And we will see more measles than we should ever see."

As Liam played with his parents across the kitchen table, Ms Requillo said the answer is simple.

"Get out there and get vaccinated," she said.

"It's really important."