Experts are calling for parents to be vigilant as cases of a dangerous lung virus grow in NSW.
Bronchiolitis, an infection of the airways caused by various viruses, happens most often in the colder months.
The NSW Health respiratory surveillance report has recorded instances of the illness in babies and toddlers aged 0-4 years old.
In the last four weeks, cases of bronchiolitis jumped from 359 to 621 cases.
Hospitalisations also increased — more than 43 per cent of babies taken to hospital with the illness last week were admitted.
Paediatric infectious disease expert David Isaacs said the issue is "serious" but rarely fatal.
"The fatalities are very few, nowadays," he said.
"There were years where we had 700 admissions of [the virus] in a season, what we're seeing is quite large numbers compared with other years."
During the first COVID-19 outbreak in the state, NSW saw zero cases of the respiratory syncytial virus (RSV) which can develop into bronchiolitis.
Professor Isaacs said the cases are now on the increase, and many more are unreported in the community.
"Only the most severe would present to a hospital, there would be many more being cared for at home that maybe don't have such a bad reaction."
On the plus side, Professor Isaacs has seen the combination of COVID and bronchiolitis and said it is "not as bad as expected".
"We had a young child present with COVID and RSV just this week and they are handling it well, it doesn't seem to be as bad as, say, RSV and the flu at the same time."
Professor Isaacs said fluids, eating and breathing should be monitored in sick children at home, and immediate action should be taken if the child's lips turn blue.
"That means go straight to your nearest emergency department," Prof Isaacs said.
"The vast majority of children will get better. It is very unusual for a child to get extremely sick unless they have underlying problems, are born very early pre-term babies, those with Down syndrome or a cancer patient."
Dr Brendan McMullan from the University of NSW said a massive spike in influenza cases was concerning as RSV circulated.
"We are seeing more respiratory viruses this winter in children than we have in the last couple of years, and since there is no real vaccine for bronchiolitis, parents need to keep an eye out for warning signs."
NSW Health is advising parents to get a flu shot, for children aged six months or older.
"Given the low exposure to flu over the last two years, children (particularly those under 2 years) may be more vulnerable to catching flu this year," a NSW Health spokesman said.
