Noah Lyles revealed he had Covid after collecting Olympic 200 metres bronze on Thursday night.
The American sprinter said he tested positive around five o'clock on Tuesday morning and woke up in the middle of the night "with chills".
Lyles, the Olympic 100 metres champion said he quickly quarantined in a hotel, but insisted on competing in the final won by Botswana's Letsile Tebogo.
The 27-year-old, who was hoping to become the first man since Usain Bolt to win both Olympic sprint titles, said the illness had affected his performance at the Stade de France.
He said: "I do have Covid. I tested positive around 5am on Tuesday morning. Woke up in the middle of the night feeling chills, aching, sore throat and those were a lot of the symptoms I've always had right before getting Covid.
"We tested it and it came back positive so we quickly quarantined in a hotel near the village and get me on as much medication as we legally could to make sure that my body was able to keep the momentum going.
"I still wanted to run, we decided it was still possible, so we just stayed away from everybody and tried to take it round by round. I knew if I wanted to come out here and win I would have to give everything I've had from the get-go. I didn't have any time to save energy. That was the strategy.
"It definitely affected my performance. I am more proud of myself than anything. Coming out and getting the bronze medal with Covid.
"We tried to keep it close to our chest. The medical staff, my coach and my mum knew.
"We didn't want everybody to go into a panic. We wanted to be able to compete. We wanted to be able to make it discreet as possible and you don't want to tell your competitors you are sick.
"Why would you give them an edge?"
The American world champion at the distance was hoping to become the first man since Usain Bolt at the Rio 2016 Games to secure both the 100m and 200m titles.
He had managed to win 100m gold in Paris, but this time could not compensate for what was again the slowest reaction time of the pack.
Tebogo crossed in 19.46 for his first Olympic title and the United States' Kenny Bednarek snapped up silver.
Lyles was slowest off the blocks in 0.173 seconds but had clawed his way up to third by the 20-metre mark and remained in bronze medal position for the rest of the race, finishing in 19.70.
Tebogo and Bednarek battled for their positions, the American holding the lead just after the halfway point, but it was the Botswanan's race to lose from the 120 metres mark and he never looked close to letting go.
Tebogo, 21, became the first man from his country to win an Olympic 200m final and dedicated the medal to his mother, Elizabeth Seratiwa, who passed away last May.
It is a second 200m bronze for Lyles, who had also hoped to compete in the men's 4x100m and 4x400m relays in Paris, though the latter was a long shot if he were in a position to secure four Olympic golds in one Games - a feat Bolt never achieved.
Lyles admitted he was feeling "quite light-headed" and experienced chest pain following the 200m final.
He said: "This is by far the best day I've felt out of the last three days. I still wouldn't say I am 100 per cent but I am closer to 90 or 95.
"I only had one shot and then I will be down for a while. Me and my therapist said 'it's one and done, there is no holding back'.
"We haven't decided on the relay yet.
"I'll talk to the coaches and to be honest, I'm gonna be very honest and transparent with them, and I'm gonna let them make the decision.
"I believe that no matter what happens this four by one team can handle anything and everything. I've been with them in practice. They are more than capable to come out with a win and even break as many records as they take their mind to."
A USA Track and Field statement read: "We can confirm that Noah Lyles tested positive for Covid-19 on Tuesday, August 6th. In response, the USOPC and USATF swiftly enacted all necessary protocols to prioritize his health, the wellbeing of our team, and the safety of fellow competitors.
"Our primary commitment is to ensure the safety of Team USA athletes while upholding their right to compete. After a thorough medical evaluation, Noah chose to compete tonight. We respect his decision and will continue to monitor his condition closely.
"As an organization, we are rigorously adhering to CDC, USOPC, and IOC guidelines for respiratory illnesses to prevent the spread of illness among team members, safeguarding their health and performance."
