Forty people associated with the World Championships have reported illness, with three cases of norovirus identified, Public Health England has announced.
Illness has affected athletes and officials at the championships, taking place at the London Stadium until Sunday.
"PHE has been notified of a confirmed outbreak of norovirus among people associated with the World Athletics Championships," PHE London deputy director for health protection Dr Deborah Turbitt said.
"We have so far been made aware of approximately 40 people reporting illness and three of these cases have been confirmed as norovirus by laboratory testing."
London 2017, the championship organisers, announced on Monday night that several competitors - staying at the same official team hotel - had suffered gastroenteritis.
A spokesperson for the Tower Hotel on Tuesday morning insisted it was "not the source of the illness".
The latest update from Public Health England came on Thursday afternoon, ahead of the men's 200m final which will feature Isaac Makwala of Botswana.
Makwala overcame illness, quarantine and two races in little more than two hours on Wednesday to qualify third quickest for the final.
Makwala had been given medical dispensation to withdraw from the 200m heats on Monday night.
Without a valid reason for pulling out, he could have been disqualified from the 400m final - an event he was forced to miss under protocols imposed by Public Health England.
His 48-hour quarantine expired on Wednesday afternoon and the IAAF accepted a request from Botswana to allow Makwala to try to qualify for the shorter event.
He had to run a solo time trial to advance to the semi-finals at 2055BST, where he was an additional competitor and qualified for the final.
But the manner of his exclusion from Tuesday night's 400m final still rankles.
"Yes, I'm running with anger. I still want my 400. That's my race," Makwala told the BBC.
"I'm still running heartbroken."
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