Organisers said gastroenteritis had been reported by team members in one of the official team hotels.
Isaac Makwala, a Botswana sprinter, said he did not start the 200m event due to food poisoning.
“According to IAAF medics I am apparently suffering from food poisoning which has affected several other athletes in the athletes' hotel,” he posted on Facebook.
Thomas Barr withdrew from the semi-final at the IAAF World Championships due to the vomiting bug.
The Irish hurdler said he was ‘bitterly disappointed’ to be forced to withdraw.
“I wasn't feeling great yesterday evening and later in the night I was hit with a bad bout of gastroenteritis.
"My whole year has been focused on the World Championships. The support yesterday was just brilliant and to not be able to go out and compete…for Ireland is beyond disappointing."
Dr Deborah Turbitt, PHE London deputy director for health protection, said it had been notified of a outbreak of norovirus.
"We have so far been made aware of approximately 30 people reporting illness and two of these cases have been confirmed as norovirus by laboratory testing. PHE has been working closely with the London 2017 organisers and the hotel to provide infection control advice to limit the spread of illness."
Norovirus is often caught through contact with someone carrying the virus or by touching contaminated surfaces or objects.
London 2017 organisers said there had been cases of gastroenteritis reported by team members staying at one of the official team hotels.
"Those affected have been supported by both team and LOC [Local Organising Committee] medical staff. In addition we have been working with Public Health England to ensure the situation is managed and contained.
"As a result, further advice and guidelines have been issued to team doctors and support staff - standard procedure for such an occurrence where a number of teams are occupying championship accommodation."