"He was real sick," manager Dusty Baker said. "They gave him some IVs so hopefully he'll be OK tomorrow. Whatever he had in his system hopefully he got it out.
"We've kind of got a sick ward here in the last couple weeks. I don't know who the carrier is, but I'm hoping whoever he is, he's well and it can end right here."
Harper had been in the original starting lineup, batting third and playing right field. Instead, Chris Heisey played right field, Trea Turner manned center field while Wilmer Difo started at second base against left-hander Robbie Ray.
This illness comes with Harper in one of the worst slumps of his career. In the last 19 games, he is batting just .123 (8-for-65) without a multi-hit game. His overall batting line is down to .235/.380/.443 with 20 homers and a .823 OPS.
Jamal Collier covers the Nationals for MLB.com. Follow him on Twitter at @jamalcollier. This story was not subject to the approval of Major League Baseball or its clubs.