Navalny is said to have had a fever of 38.1 degrees, back pain and a cough on Monday when he was transferred to the prison's hospital ward.
Three out of fifteen of the other prisoners in his ward are said to have suffered from suspected tuberculosis.
On Tuesday, Navalny's doctor, Anastasia Vasilyeva, tried to get into prison to be able to care for the opposition politician, but was stopped by police.
Are you afraid he's going to die here?
- It is clear that I am afraid, I think everyone here is afraid of it, Vasiljeva says to SVT.