Athletics' governing body has defended its decision to deny a World Championship medal favourite entry to London Stadium amid attempts to control a "very virulent" norovirus outbreak.
Botswana's Isaac Makwala, 30, had hoped to run in Tuesday's 400m final, but was barred from competing.
His country's Olympic boss, Falcon Sedimo, said that was "disturbing".
But the IAAF said it was under instruction to quarantine athletes who showed symptoms of the virus.
It acknowledged Makwala's absence was "a sad case" but said its medical staff examined the athlete and notes taken by a doctor showed he had been vomiting over an 18-hour period.
Pam Venning, head of medical at the IAAF, told BBC Sport: "I have to trust my doctors. My role is to ensure the healthcare of all the athletes here and it's a very infectious and very virulent disease."
Makwala had been considered the main threat to Olympic champion and world record holder Wayde van Niekerk, who retained his world title by winning Tuesday's race.