Ronaldo has been training hard to regain full match fitness
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Striker Ronaldo was forced out of Brazil's training session on Thursday because of illness.
The Real Madrid star trained normally on Wednesday after recovering from the blisters that forced him off the field in Sunday's friendly with New Zealand.
"He woke with a fever and is receiving medication," said a statement on the Brazil Football Federation website.
World Cup holders Brazil are building up to their opening match of the tournament against Croatia on 13 June.
But Ronaldo's season has been marred by niggles.
The 29-year-old, who was the top scorer at the 2002 finals, with eight goals, missed the end of the Spanish domestic season because of injury.
He has scored five goals in Brazil's three warm-up matches since they arrived in Europe on 22 May, against Fluminense's Under-20 squad, Swiss side FC Lucerne and New Zealand.