Harper has been battling the virus for the last few days and yesterday marked the first game that he has not appeared in since his promotion to the big leagues.
Harper and the Nats will face 26-year-old righty Mark Rogers, who will be making his first major league start since 2010.
Rogers got called up from Triple-A to start today's game after the Brewers traded Zach Greinke earlier this week.
Gio Gonzalez gets the ball for the Nats as he looks to earn his 14th win of the season.
Here's today's Nationals lineup:
2B Steve Lombardozzi
RF Bryce Harper
3B Ryan Zimmerman
1B Adam LaRoche
LF Michael Morse
SS Danny Espinosa
CF Roger Bernadina
C Jesus Flores
P Gio Gonzalez
Update: The Nationals announced that they've unconditionally released outfielder Xavier Nady, who was designated for assignment last week.
Nady hit .157 with three home runs and six RBIs with the Nationals this season.