Valencia's Argentina midfielder Pablo Aimar is suffering from a form of viral meningitis, the Primera Liga club have announced.
The 26-year-old, a key player in Argentine coach Jose Pekerman's World Cup plans, was admitted to hospital on Wednesday after complaining of fever and headaches.
Valencia said Aimar was expected to remain in hospital for up to 10 days.
"At the moment the patient is stable and his initial condition has improved," neurologist Jose Tatay Ramirez told the club's website. "He will need to remain in hospital to see how he evolves and responds to the prescribed treatment."
Tatay said Aimar has a benign form of meningitis which would not have any permanent consequences and was not contagious.
Valencia are in second place in the Primera Liga, level on points with Real Madrid and 11 behind leaders Barcelona.
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