A lack of discipline among residents has been blamed for a town's "home-brewed" tripling of Covid-19 cases.
Wymondham in Norfolk reported 143 new cases in the week to 14 November, up from 37 the previous week.
The infection rates in South Norfolk overall have more than doubled from 92.3 to 225 per 100,000 people.
South Norfolk Council leader John Fuller said it was a "wake up call" to those who had not isolated at home after developing symptoms.
"This is a home-brewed outbreak," he added.
"We don't have any obvious sources of infection in the town - there are no food factories or universities, the hospital is far away and yet this has bubbled up inside people's homes.
"People have not been self-isolating if they've got symptoms or while waiting for test results and they've passed it on to their families.
"What's happened in Wymondham could happen anywhere."
'Stick to rules'
The town had 21 cases up to the end of April, with no new infections recorded between May and October.
There were 56 infections in Wymondham West in the week to 14 November, up from just four the previous week.
Over the same period, Wymondham East and Spooner Row - a neighbouring village - recorded a rise of 33 to 87, the 32nd highest number of cases in a neighbourhood in England.
Director of public health in Norfolk, Dr Louise Smith, said the Wymondham outbreak was a "striking example" of community spread.
"If we can't tie down infections to a certain source it is much harder to take action," she said.
"The feedback and the messages we are getting is that people are not sticking to the essential rules.
"It is essential during this lockdown that you only go out when it is necessary for work or education or to get essentials - it's the only way we're going to get on top of this community spread."
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