Isaac Makwala of Botswana competes in the men's 400m semi final during the 16th IAAF World Athletics Championships London 2017 at the London Stadium on Aug 6, 2017. [Photo/VCG]

Several Botswana, German, Canadian, Irish and Puerto Rican athletes staying at the Tower Hotel, near Tower Bridge, have been taken ill over the last few days, with some put into effective quarantine and others forced to miss their events.

The victims included Botswana's Isaac Makwala, who was ordered by the global athletics body, IAAF, to withdraw from Tuesday evening's 400 meters final, where he had expected to be the lead challenger to world record holder Wayde van Niekerk.

Thirty German competitors arriving on Tuesday, as well as Olympic javelin champion Thomas Rohler who arrived on Monday, have been moved to other hotels.

"It is purely a precautionary measure," German team spokesman Peter Schmitt said.

Competition organizers said on Monday that the illnesses were a result of gastroenteritis, but public health officials said on Tuesday that laboratory tests have confirmed two cases of norovirus among approximately 30 illness victims.

Norovirus, sometimes called "the winter vomiting bug," is easily spread, partly because the virus can survive for several days outside the body, Britain's National Health Service says.

"The main issue facing the organizers will be one of trying to attain swift containment, which will be pretty challenging due to the nature of the virus," Shirley Kirnon, a lecturer in Infection Control at Birmingham City University, said.

"It is highly infectious. For those affected, symptoms of nausea, vomiting and diarrhoea can occur within a relatively short period of time; approximately 12-48 hours after exposure."

Tower Hotel, used annually as the base for the London Marathon, said in a statement: "We have worked collaboratively with the Environmental Health Officer and the IAAF to investigate the origins of the illness and can confirm that the hotel was not the source.

Reuters