Dorset Council says that cases of Strep A are 'higher than usual for this time of year' as the infection spreads across the county.
Parents are being urged to stay alert after a serious case of the disease was confirmed at a Weymouth nursery on Tuesday.
Dorset GP surgeries and the county hospital in Dorchester have said they are 'extremely busy' as they deal with ill children.
The infection has claimed the lives of several youngsters in the UK and can cause severe illness, with health officials also reporting a surge in scarlet fever cases, which is linked to Strep A.
The infection survives in throats and hands and is spread through sneezing, kissing and skin contact, and on rare occasions can cause severe disease if it gets into an unknown part of the body.
These infections are known as invasive Group A Streptococcal disease, which a nursery child at Weymouth's Holy Trinity Church of England Primary School and Community Nursery was diagnosed with on Tuesday.
NHS Dorset said these cases 'are rare' but warned that parents should be aware of what to look out for and should call 999 or go to A&E if they think their child is seriously unwell.
There are almost 50 known cases of serious Strep A infections in South West England, according to from figures from the UK Health Security Agency.
Symptoms of invasive Strep A include fever, severe muscle aches and localised muscle tenderness, and parents are advised to seek medical help if they or a child develop any of these.
Elsewhere, several cases of scarlet fever have been confirmed at other Weymouth schools, including Wyke Regis Primary Federation and Bincombe Valley Primary School.
Bridges Medical Centre, in Commercial Road, Weymouth, texted patients on Wednesday to say they are treating 'an increased number of ill children'.
"Please be aware that we are extremely busy at the moment - please be patient with our staff," the message added.
Royal Crescent and Preston Road Practice, in Crescent Street, Weymouth, added it was experiencing an unusually high demand of telephone calls.
"Please can you be patient when trying to contact us, we will try to answer all requests for advice if you or your child is unwell," it said.
"Parents should remain vigilant particularly if your child has an underlying condition which affects their immune system."
Dorset County Hospital also put out a plea on Tuesday, urging people with minor injuries to avoid visiting as it was also experiencing a surge in patients this week.
Deputy director of public health for Dorset, Rachel Partridge, said that figures for both Strep A and scarlet fever are higher than expected for the time of year.
"For most people these infections result in mild illness and in Dorset we have a well-established process in place to reduce the spread of infectious diseases," she added.
“We work closely with families, schools and partners and continue to share information with schools about any measures they need to take, including advice for parents.
“It’s important to remember that during the winter months, there are lots of viruses circulating that cause sore throats, colds and coughs, and most people will get better at home without needing medical care.
"However, if you notice the symptoms and have concerns you should contact NHS 111 or your GP.”
GP and chair of the Dorset GP Alliance, Dr Forbes Watson, added: "We recognise that parents are concerned about the health of their children and would urge them to visit the NHS Dorset website where they can find links to reliable and up to date information.
“Being winter there are lots of bugs circulating and most of these will clear up on their own, but if you recognise any of the symptoms, please use 111.nhs.uk, call 111 or speak to someone at your local GP practice.
“If you do not have access to the internet and have concerns, please call 111 in the first instance or speak to your GP practice.”
The council added that 'there is currently no evidence to suggest a new strain is circulating' and the increase in Strep A is most likely related to the return of social mixing following the COVID-19 pandemic.
It added that good hand and respiratory hygiene is important for stopping the spread, as the disease is highly infectious and spread by close contact with an infected person.
Children with scarlet fever should stay home for at least 24 hours after starting antibiotics to avoid spreading infection to others, the local authority advises.