A new strain of group A streptococcus, which is capable of producing significantly more toxin, is spreading in England and Wales, scientists say.
Strep A causes a range of infections, from a sore throat to scarlet fever.
The new strain's emergence, reported in The Lancet Infectious Diseases, coincides with an increase in rare but potentially deadly invasive infections.
However, it does not explain the mysterious surge in scarlet fever cases during the past five years.
And there is no direct evidence the new strain causes more serious infection than other strains, experts say.
Scarlet fever is a very contagious infection that mostly affects young children.
The number of children with scarlet fever soared in 201
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The latest figures showed there were 1,500 invasive cases in England in 2018-19 – 8% higher than the average for the previous five years.
A team from Imperial College London analyzed the DNA, or genetic code, of stored strep A samples from patients to see if a change in the bacterium could explain the rise.
Strep A is made up of several "families" known as emm types. Emm1 has been linked to invasive cases in the past and the study showed it had mutated to form the new strain.
"We found no trace of it in the UK until about 2010," one of the researchers Prof Shiranee Sriskandan told the BBC News.
"And it took off between 2011 and 2013, so by 2016 it represented 80% of emm1 strains – it's taken over its own family."
The mutated strain is capable of producing nine times more toxin (streptococcal pyrogenic exotoxin A) than the previous version.
This is the toxin that actually causes the symptoms of scarlet fever – flu-like symptoms, sore throat, swollen glands and a rash.
However, Prof Sriskandan told BBC News: "This study does not offer any answers to why scarlet fever came back in 2014."
There is also no evidence, she says, that the toxin is causing the invasive cases of the disease.
The new strain has largely been confined to the UK. There has been one case in Denmark and another in the US.
"It's important because although invasive disease is very rare due to strep A, it has quite a substantial impact on the people it affects," said Prof Sriskandan.  "Mortality from invasive disease is between 15% and 20% – these are very high mortality rates."
Scarlet fever – because it is easily noticed and cases have been reported to Public Health England – is a good indicator of how much strep A is circulating.
In the past five years, England has seen the biggest surge in scarlet fever since the 1960s.
In 2013 there were about 4,000 cases, reaching 15,000 in 2014 and 19,000 in 2016. The number of cases is now coming down again but remains relatively high.
Prof Mark Walker, from the University of Queensland, Australia, said: "An unprecedented global resurgence of scarlet fever and severe invasive group A streptococcal infections has been seen in the past few decades. "
China, South Korea and Hong Kong have all recorded similar spikes in cases and Prof Walker said there was an "essential need" to monitor group A strep infections around the world.
"The report sends out an important warning [that] recently emerging scarlet fever group A strep strains have enhanced invasive potential which may have profound implications for the future," he said.
Prof Jimmy Whitworth of the London School of Hygiene and Tropical Medicine said: "This important study gives us a plausible clue to the worrying recent increase in cases of scarlet fever in children in England."
A spokeswoman from Public Health England said: "Despite the rise in scarlet fever cases over the last five years, it remains a typically mild illness, readily treatable with antibiotics to reduce the risk of complications and spread to others.
"We remind parents to be aware of the symptoms of scarlet fever and to contact their GP for assessment if they think their child might have it. "
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