London - Botswana star Isaac Makwala was withdrawn from the world 400m final on Tuesday after an outbreak of the norovirus bug at a hotel hosting athletes at the IAAF World Championships.
"Isaac Makwala was withdrawn from the men's 400m (Final) due to a medical condition on the instruction of the IAAF medical delegate," world athletics' ruling body said in a statement.
Makwala was withdrawn from the first round of the men's 200m on Monday for the same reason, but insisted Tuesday he was ready to run the 400m, scheduled for 2050 GMT, before the IAAF stepped in.
Public Health England said 30 athletes and support staff had been affected at a central London hotel, with two cases confirmed as being the norovirus bug.
Dr Deborah Turbitt of Public Health England (PHE) said: "PHE has been notified of a confirmed outbreak of norovirus among people associated with the World Athletics Championships.
"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 close contact with someone carrying the virus or by touching contaminated surfaces or objects.
Norovirus, which brings on diarrhoea and vomiting, is rarely serious, with most people making a full recovery within one or two days, without treatment.
However, Shirley Kirnon, Senior Lecturer in Infection Prevention and Control at the School of Health Sciences at Birmingham City University, warned the organisers faced a race against time to prevent it spreading like wildfire.
"The main issue facing the organisers will be one of trying to attain swift containment, which will be pretty challenging due to the nature of the virus," she said.
"It is highly infectious and with vast numbers of people -- athletes, site personnel and visitors -- in such close proximity, exposure to affected individuals cannot be contained easily."
Kirnon added that it was crucial those affected were kept away from presently healthy individuals.
"In terms of public health, the emphasis should be to limit the exposure to others," she said.
"This involves limiting social integration and movements of affected individuals; for athletes staying in the rooms whilst having active symptoms -- remaining within the designated camp, and visitors with symptoms staying away from public areas."