An investigation has revealed why the deadly meningitis B outbreak in Kent was so dangerous.
Two teenagers died and nine needed intensive care in March - with 21 people in total diagnosed in just a week.
Medics said all patients contracted a strain of invasive meningococcal disease (IMD).
A Canterbury nightclub popular with students was linked to the outbreak.
Scientists have concluded the bacteria involved picked up small pieces of DNA from other less harmful bacteria found naturally in the throats of many heathy people.
The process is known as horizontal gene transfer.
These changes altered how the bacteria interacted with human cells, according to research by the UK Health Security Agency and the University of Oxford.
The scientists say it made the strain very effective at causing severe illness and may have affected how it was recognised by the immune system.
Such stains are unpredictable and can emerge suddenly, suggesting similar outbreaks could occur again, according to the UKHSA.
Pathogen genomics expert Dr Charlene Rodrigues said it showed how "dramatically these bacteria can change, sometimes acquiring new traits from harmless bacteria circulating nearby, that make them more likely to cause disease".
She said "the very same genetic changes that made this strain so dangerous may also explain why it hasn't continued to spread.
"But that is not something we could have predicted and these genetic events can happen again."
A special meningitis B vaccination programme was launched in the area following the outbreak and experts recommended in July that teenagers should be given the jab when they're about 15.
The proposal was separate to another initiative this summer to vaccinate about a million Year 13 students and those heading to university.
Meningitis is an infection of the protective membranes that protect the brain and spinal cord.
More from Sky News:
Meningitis B: The symptoms and treatments
If not treated quickly, it can lead to life-threatening sepsis and permanent damage to the brain or nerves.
Symptoms can include a temperature, severe headache, vomiting, sensitivity to light, confusion, drowsiness, cold hands and feet, severe muscle pain and a rash that doesn't fade under pressure.
The infection spreads through saliva and spit but close and lengthy contact is usually needed, such as kissing, sharing vapes, or living in close proximity.
Students can be more vulnerable through co-living arrangements and attending large social events.
(c) Sky News 2026: Scientists reveal why Kent meningitis B outbreak was so deadly
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