Isaac Makwala was denied his place in Tuesday's 400 metres final to protect other athletes from contracting an illness, says IAAF head of medical services Pam Venning.
After being pulled from the 200m heats amid an outbreak of gastroenteritis at a team hotel in use at the IAAF World Championships on Monday, Makwala was withdrawn from the 400m line-up by world athletics' governing body just hours before the scheduled start time.
The 30-year-old was denied entry to London Stadium for the race and Botswana National Sports Commission chief executive Falcon Sedimo criticised the IAAF for a lack of communication on the issue.
A statement from the IAAF ahead of the final explained a 48-hour quarantine procedure recommended by Public Health England - who on Tuesday revealed 30 people had reported illness and two were confirmed as norovirus following laboratory testing - was behind the refusal to admit Makwala to the arena.
Venning gave further details on how the decision was reached following an examination of the athlete on the previous day.
Asked if any other member of the Botswana delegation was present for the tests conducted on Makwala, Venning told the BBC: "No, because the athlete came into the medical centre of his own accord and was seen by one of our volunteer doctors and examined.
"When he was told he had a gastroenteritis disease he tried to leave but one of my other senior members of the staff brought him back in because we have an outbreak of gastroenteritis in the hotels and it is highly infectious.
"He'd taken a history, examined him, pulse, respiratory and taken a history and the history is very clear from this gentlemen that he'd had very similar symptoms to all the other athletes that have also been classed as having this gastroenteritis disease.
"My responsibility is to ensure the healthcare of all the athletes here and it's a very infectious and very virulent disease.
"We feel very sorry for the athletes that have to be withdrawn from competition but we have a responsibility to all the athletes and if we allow them all - the call room is a close, tight community and we have to protect the other athletes."
Posed with a version of events from the Botswana team that denied Makwala had reported vomiting over the weekend, Venning replied: "That is not the history we have. He'd been vomiting from 10pm the evening before and he had vomited at two o'clock that afternoon [Monday] and that is the history he gave to our doctor and that's what we wrote down in the records.
"So as far as we are concerned he is staying in the same hotel, has the same symptoms as other athletes that have also been quarantined from mixing with other people.
"My advice from Public Health England was that we didn't need to take any [blood] samples because the symptoms are the same, the length of the vomiting and other aspects are the same, so we didn't need to take any other specimens."
Wayde van Niekerk triumphed in Makwala's absence, with the Botswanan's only remaining chance for a medal will come in the 4x400m relay this weekend.