The oldest female skier to medal at a championship, her last a downhill bronze in 2019 at age 34, posted a story on Instagram showing her wearing an inhaler mask and the words "current state...could be better."
"I have a bad cold or flu but don't worry, I wouldn't let something like that prevent me from racing after everything I've gone through to get here," Vonn said in a post from the Austrian resort of Saalbach.
"I guess the competitor in me is still there."
Vonn, an eight-time world championship medallist and 2010 Olympic downhill champion, calculated it had been 2,187 days since her last championship race.
The 82-time World Cup race winner finished fourth in a Super-G race in St. Anton, Austria, last month and sixth in a downhill in the same resort.
She is due to start with bib number 30 on Thursday.
"I feel lucky to get this opportunity but also thankful that I was able to earn my way here with a body that is finally working again," said the two-time world champion before the sickness update.
"I know I still have the ability to succeed in this sport but will all the pieces of the puzzle fit together tomorrow? I don't know.
"What I do know is this: I will give it my all, I will not take it for granted and I will have fun... and if everything doesn't fit together tomorrow, that's OK too. I still have 368 days to figure it out."
Vonn has made skiing at the 2026 Milan-Cortina Winter Olympics, which starts next February, the main goal of her comeback.