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06/04/09 5:39 PM ET

Willingham fighting viral infection

Nationals outfielder doesn't know when he'll return

WASHINGTON -- Nationals left fielder Josh Willingham said Thursday afternoon he was unable to play this week because of a viral infection.

Willingham said he started getting sick after Sunday's game against the Phillies, but felt worse when he woke up the next morning. At first, the Nationals didn't know what kind of illness Willingham had, so he took some blood tests to determine if he had a viral or bacterial infection. The doctors concluded that he had a viral infection.

"It's running its course. I feel a little bit better. I don't know when I'm going to play. There's a lot of weakness and soreness in the muscles," he said.

The viral infection comes at a bad time for Willingham, for he has been one of the Nationals' best power hitters, slugging eight home runs in his past 24 games.

"Nobody wants to get sick, especially with something you really don't know what's going on," Willingham said. "You bang your knee or something in a ballgame as an athlete, you can take that."

In other news, third-base coach Pat Listach will not be on the coaching lines Saturday against the Mets. He will be attending his daughter's high school graduation. Hitting coach Rick Eckstein will take over the third-base coaching duties that day.

Listach will return to the Nationals on Sunday.

Bill Ladson is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.