The coronavirus outbreak which has closed down a Norwich firm employing almost 800 people is one of more than 70 currently affecting Norfolk businesses.
Dr Louise Smith, Norfolk's public health director, said that the outbreak at Ingram Micro UK was the largest of those which health bosses were dealing with.
A total of 190 people, almost a quarter of the workforce at the company, based at Vulcan Road North, near Norwich Airport, have tested positive.
The firm specialises in mobile phone and computer logistics and employs 795 staff.
Norfolk County Council, which instigated testing at the site this week, said the firm, formerly known as Avono, has now "voluntarily closed" for the next 10 days.
Staff who have tested positive are isolating and the rest of the workers have been asked to isolate for 10 days.
And Dr Smith said it was one of more than 70 outbreaks - defined as two or more cases linked to a specific location - across Norfolk.
She said: "Even though we are in lockdown, the number of outbreaks we are dealing with in businesses is large.
"We have got 72 ongoing outbreaks and it is getting challenging.
"They vary in size, with Ingram Micro UK by far the largest, with the vast majority under 15 members of staff.
"The challenge we have is that, because the numbers in the community are so high, test and trace and contact tracing is not really effective, which is why we are in lockdown.
"One of the problems we have is separating the signals that a case is linked to a location from the general noise.
"That makes it difficult to know if the numbers which we are seeing from a business are significant or not.
"The numbers are so high that it's hard to pin down whether cases are linked that location."
The council has recently started rapid testing at two sites in Norfolk, with more to follow.
Lateral flow tests, which can return a result within 30 minutes, are being used in Great Yarmouth and King's Lynn.
They aim to identify asymptomatic cases among key workers, so they can get them to self-isolate.
Other people who have coronavirus symptoms and feel unwell should still get tests in the usual way and self isolate.
Tests can be booked by calling 119 or visiting www.nhs.uk.
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