Navalny's allies said the 46-year-old was placed in a cramped punishment cell for 15 days on December 31, the 10th time he had been sent there in the space of five months for misdemeanours such as washing his face at the wrong time or failing to button up his prison uniform.
Navalny posted on social media via his lawyers this week that authorities had deliberately infected him by placing a sick man next to him as a "bacteriological weapon".
His allies say there is a flu outbreak in the prison colony where he is being held east of Moscow, and he has a bad fever and cough.
His wife Yulia appealed to prison authorities via Instagram.
"Are you human? You have parents and children waiting for you when you come back from work. What's going on in your head? How do you live, rejoicing that you deliberately infected a man and you don't treat him or pass on any medicines?" she asked on Wednesday.
Russia's federal prison service, FSIN, did not immediately reply to a Reuters request for comment.
Navalny's supporters said a legal hearing about his prison conditions went ahead on Wednesday despite his request for a delay. The proceedings' outcome was not clear.
"Navalny looks exhausted. He is really ill, and the prison colony is refusing him the most basic and necessary medicines. They are just slowly killing him in prison," his ally Lyubov Sobol said on Twitter.
Navalny, the most prominent domestic critic of President Vladimir Putin, survived an attempt to poison him in 2020 with what European scientists said was a deadly nerve agent. The Kremlin denies that conclusion and trying to kill him.
He recovered but his health has been a concern ever since. He was imprisoned after returning home in 2021 and is serving 11-and-a-half years' jail on charges including fraud and contempt of court.
Navalny, his allies and Western governments and rights groups say he is the victim of trumped-up charges designed to silence him.
