Jailed Kremlin critic Aleksei Navalny has been moved to a sick ward after complaining of a cough and temperature, the Izvestia newspaper has reported.
Earlier in the day, Navalny said in an Instagram post that a third prisoner in his quarters had been sent to the hospital with suspected tuberculosis.
In the post, Navalny said prison doctors had officially diagnosed him with a "severe cough" and a temperature of 38.1 degrees Celsius, which indicates a slight fever.
President Vladimir Putin's most prominent critic, 44, is currently incarcerated in Correctional Colony No. 2, about 100 kilometers from Moscow, which is known as one of the toughest penitentiaries in Russia.
Navalny said his prison unit consists of 15 people, three of whom have been hospitalized with suspected tuberculosis.
Tuberculosis is a potentially serious infectious disease that mainly affects the lungs and is spread from person to person through tiny droplets released into the air mainly via coughing and sneezing.
It has largely been eradicated in developed countries and a person with a healthy immune system often successfully fights it.
In his April 5 post, Navalny said his prison unit had been malnourished with clay-like porridge and frozen potatoes. He is currently on a hunger strike to demand better conditions.
Malnutrition and weight loss undermine an immune system's ability to fight tuberculosis.
Navalny had previously complained of acute back and leg pain as well as not being allowed to sleep by his guards.
Navalny criticized recent news reports by state-owned media that he is serving in a prison with comfortable conditions.
He invited state media correspondents to come stay the night in his prison with tuberculosis-infected cellmates.
Russian police arrested Navalny in January upon his return from Germany on charges of violating his parole, sparking large-scale protests.
The anti-corruption fighter had been recuperating in Berlin for several months after being poisoned with a military-grade nerve agent.
Navalny has accused agents of Russia's Federal Security Service of attempting to assassinate him with the poison.
A Moscow court in February found him guilty of violating the terms of his parole from an older embezzlement case that is widely considered to be politically motivated.
His suspended 3 1/2-year sentence was converted into jail time, though the court reduced that amount to 2 1/2 years for time already served in detention.
Navalny's imprisonment has drawn a chorus of international criticism, with the United States and its allies demanding his unconditional release and vowing to continue to hold those responsible for his poisoning to account.