Ronaldo went down with dengue fever in January 2012 and posted a picture of himself on Twitter hooked up to a drip in hospital alongside the message: "Good morning to all those who began the New Year with dengue".
The former Real Madrid and Barcelona forward was said to have been rushed to intensive care in Ibiza after being struck down with pneumonia, eventually being transferred to a private hospital, Clinica Nuestra Señora del Rosario.
Hospital sources have said he is still in intensive care but making good progress.
This story will be updated when more information on Ronaldo's condition is known.More news: Alisson set for debut against West Ham United
He is a regular traveler to Ibiza and also owns a house in the island.
He is no stranger to illness, however.
In his latter years he battled with overweight because of a thyroid problem.
After winning everything there is to win in the game he retired in 2011.More news: House of Frazer bought by Mike Ashley
Symptoms - which include a cough and difficulty breathing - can develop suddenly over 24 to 48 hours. It can be potentially life-threatening if severe and is particularly unsafe for the elderly or those with pre-existing conditions.
The crowning achievement of his career was his display in the World Cup final in 2002. He has won the elusive trophy in 1994 and 2002 and has scored 62 times for Brazil. Throughout a prosperous career, he scored 62 goals in 98 worldwide appearances for Brazil.
He picked up the Ballon d'Or award for the best player in the world in 1997 and 2002.
Here's hoping that he can make a speedy recovery.More news: Arsenal vs Manchester City