Former Brazil striker Ronald is reportedly in intensive care with pneumonia in Ibiza, a Spanish island.
"Friends, I had a strong bout of flu in Ibiza and had to be taken to hospital on Friday but everything is ok", the 41-year-old said in a message on his official Twitter account.
Later that night, he requested transfer to the Nuestra Señora del Rosario hospital owned by Policlinica, a private health-care company.
The paper also said that Ronaldo was still in intensive care but is making good progress.
Contacted by AFP, a hospital spokeswoman said she was unable to give any information for privacy reasons.
He is believed to have arrived in Ibiza for a summer break a few days ago.
Pneumonia is swelling of the tissue in one or both lungs and is usually caused by a bacterial infection.
He has played for Barcelona, Real Madrid, Inter and AC Milan in Europe and his career ended with Brazilian side Corinthians in 2011. Two of those were the only ones that Brazil scored against Germany in the 2002 World Cup final.
Symptoms - which include a cough and difficulty breathing - can develop suddenly over 24 to 48 hours.
During his glittering career Ronaldo won the FIFA World Player of the Year award three times, in 1996, 1997, and 2002.
It comes just weeks after the two-time World Cup victor was in Russian Federation to open the 2018 tournament, appearing on screen with the official ball and taking part in a passing routine with a local child.