Brazil striker Ronaldo was forced out of training on Thursday because of illness.
The Real Madrid forward trained on Wednesday after recovering from the blisters but he was feeling unwell on Thursday morning.
“He woke with a fever and is receiving medication,” said a statement on the Brazil Football Federation website.
World Cup holders Brazil open their campaign against Croatia on 13 June.
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.