On Thursday, Robert Griffin III was the only quarterback participating in practice, per multiple reporters on the scene.
On the positive, an illness likely isn't a long-term issue. On the negative side, he loses at least one day of valuable practice reps.
Jackson could have been in line to start if Flacco can't play Sunday versus the Cincinnati Bengals. He still might get a chance depending on the severity of the issue and the coaching staff's confidence with limited practice snaps this week. Missing Thursday, however, tilts the scale further in RGIII's favor if that eventuality occurs. There were already whispers that Griffin could start over Jackson if Flacco couldn't go.
Coach John Harbaugh said earlier in the week that Flacco doesn't need to practice in order to start.