The 41-year-old Brazilian is said to have been diagnosed with the illness, after being rushed to Can Misses Hospital on the island.
According to reports, Ronaldo, who was on holiday in Ibiza, remains in intensive care and but has been "evolving favourably".
He tweeted: "Guys, I had a bad case of influenza here in Ibiza, I was hospitalised on Friday, but everything is under control!" Tomorrow I'll be discharged and I'll go home! "Thank you all for the messages and affection!"
Brazilian World Cup victor and legendary striker Ronaldo has been admitted to an intensive care unit with pneumonia while on vacation in Ibiza, according to local reports.
The two-time World Cup champion reportedly asked for a transfer to a private health facility identified as Clinica Nuestra Señora del Rosario later that night before being taken to intensive care.
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The Pierce County Sheriff's Office said the man, who has been identified as Richard Russell , was 29 years old and "suicidal". Video showed fiery flames amid trees on the island, which is sparsely populated and only accessible by ferry.
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FILE- In this June 14, 2018, file photo Tesla CEO and founder of the Boring Company Elon Musk speaks at a news conference. The development comes days after Musk tweeted that he was considering taking Tesla private at $420 per share.
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The report from ESPN shared the scary details surrounding McNair's death, which called into question the Maryland training staff. No cause of death has been officially released, but ESPN has since learned that heatstroke is the cause of death.
At club level, he spent time with Barcelona, Real Madrid, AC Milan and Inter Milan.
Ronaldo finished his career with an impressive 414 goals, 62 of which came in just 98 appearances for the Brazilian national side.
Ronaldo is widely regarded as one of the best football players. Two of those were the only ones that Brazil scored against Germany in the 2002 World Cup final.
During his glittering career Ronaldo won the FIFA World Player of the Year award three times, in 1996, 1997, and 2002.