An outbreak of meningitis is being treated as a national incident after the deaths of a sixth-form pupil and a university student.
Thousands of students in Kent are being urged to get vaccines and take antibiotics as health officials deal with the "unprecedented" and "explosive" outbreak.
Some 15 cases have been reported to the UK Health Security Agency (UKHSA) so far. All required hospital admission.
The number of cases is expected to rise because the incubation period from infection to when symptoms appear is two to 14 days.
Around 5,000 students in university halls in Kent will be offered the meningitis B (menB) vaccine in the coming days, while four schools across the county have confirmed cases and hundreds of people are being offered antibiotics.
Experts said many of those affected had attended Club Chemistry in Canterbury between 5-7 March.
UKHSA chief executive Susan Hopkins said the outbreak "looks like a super-spreader" event with "ongoing spread" through universities' halls of residence.
"There will have been some parties particularly around this, so there will have been lots of social mixing," she added.
"I can't yet say where the initial infection came from, how it's got into this cohort, and why it's created such an explosive amount of infections.
"I can say that in my 35 years working in medicine, in healthcare and hospitals, this is the most cases I've seen in a single weekend with this type of infection.
"It's the explosive nature that is unprecedented here - the number of cases in such a short space of time.
"NHS were initially managing it as a major incident in the region but they have now increased that overlay to having a national-level oversight as well."
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While all reported cases so far have a link to Kent, according to the UKHSA, at least one person who fell ill and had links to Kent attended a London hospital.
England's deputy chief medical officer Dr Thomas Waite said: "This is by far the quickest-growing outbreak I've ever seen in my career, and I think probably any of us have seen of meningitis for a very long time.
"Whilst it remains an outbreak that is having its consequences in Kent, it is obviously of national significance."
Scientists are urgently working to establish whether the spread is caused by a possible mutant strain of menB. Of the 15 cases reported so far, four were confirmed to have that strain.
Health Secretary Wes Streeting told MPs: "This is an unprecedented outbreak. It is also a rapidly developing situation."
He said the menB vaccine has been available on the NHS since 2015 as part of routine childhood immunisations, "but clearly most students would not be vaccinated".
Four centres are open in Canterbury offering antibiotics, with 11,000 doses available on site, Mr Streeting added.
"The onset of illness is often sudden, and early diagnosis and treatment with antibiotics are vital," he said.
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