Parents have been urged to keep an eye out for signs of a winter respiratory illness in young children as infections have begun to rise.
Respiratory syncytial virus - commonly referred to as RSV - mainly spreads in winter, but is now spreading out of season after lockdowns in the colder months kept it at bay.
The virus causes mild symptoms similar to a cold and most people recover after a week or two.
However, the infection can be serious for some people, especially infants and elderly people.
Public Health England found that positive RSV samples have increased over the last five consecutive weeks from 1.2% to 8.9%.
Due to the various restrictions in place to reduce the spread of Covid-19 last winter, there were far fewer infections in younger people.
Officials said this means many will not have developed immunity and so there may be more cases this year than in a typical season.
PHE said that for the majority of children these illnesses will not be serious and they will soon recover following rest and plenty of fluids.
Health bosses said that while still at low numbers, respiratory infections in young children are expected to rise this summer and as we go into the winter months.
For this reason, the NHS is preparing for a rise in children needing treatment, officials said.
RSV is a very common virus and almost all children are infected with it by the time they are two years old.
In older children and adults, RSV may cause a cough or cold.
Some children under two, especially those born prematurely or with a heart condition, can suffer more serious consequences from these common infections such as bronchiolitis, an inflammatory infection of the lower airways, which can make it hard to breathe.
The early symptoms of bronchiolitis are similar to those of a common cold but can develop over a few days into a high temperature of 37.8C or above (fever), a dry and persistent cough, difficulty feeding, rapid or noisy breathing (wheezing).
Officials said parents should contact their GP or call NHS 111 if they notice these symptoms or if they have any concerns.
Parents are advised to dial 999 for an ambulance if their baby is having difficulty breathing, if the baby’s tongue or lips are blue or if there are long pauses in their breathing.
Dr Yvonne Doyle, medical director at PHE, said: “This winter, we expect levels of common seasonal illnesses such as cold and flu to increase as people mix more and given that fewer people will have built up natural immunity during the pandemic.
“Children under two are at a particular risk of severe infections from common seasonal illnesses.
“If a child under two is suffering from a cold, keep a close eye on their symptoms and make sure to contact your doctor if they get a high temperature, become breathless or have difficulty feeding.
“It’s important that we carry on with good hygiene habits that we’ve become used to during the pandemic, in order to protect ourselves and those around us.
“This means washing your hands regularly, using a tissue to catch coughs or sneezes and washing your hands afterwards, and staying away from others if you feel unwell.”
Health Minister Lord Bethell said: “I remember the long nights in hospital when my 8-week old daughter fought off RSV.
“The image of her tiny body plugged into those machines and gasping for air will not leave me. I would not wish those moments for anyone.
“I urge all parents and carers to be alert to the signs of RSV, particularly amongst young children. It’s a nasty bug, so watch out for it.”
