Ronaldo Nazario was admitted to hospital in Ibiza to be treated for pneumonia, with the former Brazil and Real Madrid great set to be discharged on Monday.
The 41-year-old is said to have been diagnosed with the illness after being rushed to Can Misses Hospital on the island on Friday evening.
Ronaldo, whose club career took in spells at Cruzeiro, PSV Eindhoven, Barcelona, Inter Milan, Real Madrid, AC Milan and Corinthians, is on holiday in Ibiza, where he owns a house.
At the time, the Brazilian managed to handle the indisposition humorously with his trademark smile via tweet from his hospital bed saying: "Good morning to all those who began the New Year with dengue". No further information is now available about his condition.
In his later years he battled with overweight because of a thyroid problem.
After winning everything there is to win in the game he retired in 2011.
Symptoms - which include a cough and difficulty breathing - can develop suddenly over 24 to 48 hours.
It can be life-threatening, especially among the elderly and those with underlying health conditions.
The crowning achievement of his career was his display in the World Cup final in 2002.
In total, Ronaldo scored 15 goals at World Cups and won the competition's Golden Boot award for the top scorer on one occasion.