Scarlet fever has hit a record 50-year high in England with more than 19,000 cases reported in 2016.
The disease, which is most common among children under the age of 10, has been spreading rapidly since 2014 but experts do not know why.
The number of cases tripled between 2013 and 2014, rising from 4,700 cases to 15,637, according to a new study in medical journal The Lancet.
In 2016, there were 19,206 reported cases, a record high since 1967.
Dr Theresa Lamagni, head of streptococcal surveillance at Public Health England and lead researcher on the study, said: "We are concerned - it's quite a dramatic rise."
She described the rising number of cases as "baffling" and said that no underlying causes had been identified. There was no evidence that the infection had become resistant to penicillin, which is used to treat it, she added.
"We've always seen cases of scarlet fever - it's just that the scale in the past has been much lower than [in] the last few years," she said.
In the past, the disease caused devastating epidemics. These have been avoided since the introduction of antibiotics.
Doctors are urging people to seek medical help if they notice any of the following symptoms: a rosy rash on the neck and face, sore throat, headache or fever.
At first, the rash looks like a bad sunburn but over time it can turn white and become itchy and sore to touch, "like sandpaper."
Scarlet fever is a highly contagious bacterial infection which spreads through droplets when an infected person sneezes or coughs. It can be treated with a course of antibiotics. Prompt treatment is essential to stop the disease from causing pneumonia and damage to the liver.
Anyone diagnosed with scarlet fever is urged to stay at home for at least 24 hours to prevent contagion.
"We encourage parents to be aware of the symptoms of scarlet fever and to contact their GP if they think their child might have it," said Lamagni.