Favre took a steroid pack, had an injection Saturday and missed the first hour of meetings because he couldn't get out of bed.
Favre said Saturday night the illness might be pneumonia, but the Vikings medical staff said it thinks he has been suffering from a sinus infection.
After the first week without fired head coach Brad Childress, Favre said he wants to get the magic back the Vikings offense had last year but admits he's not sure that's possible, saying, "It's not like we took the playbook and dumped it in the trash -- although maybe we should."
Favre's dedication helped the Vikings snap a seven-game road losing streak with a 17-13 win. He finished 15 for 23 for 172 yards -- and for only the second game this season, Favre did not throw an interception.
He was short one receiver, as Bernard Berrian was inactive with a groin injury.
Berrian was injured and limited to a handful of plays in Minnesota's loss to Green Bay last weekend.
Favre was more involved in creating the game plan this week with offensive coordinator Darrell Bevell, often texting suggestions or writing them on the board in the offensive meeting room.
Look for the Vikings to cut down on using motion and limiting personnel groups as they take on the host Washington Redskins on Sunday. They will re-use successful plays and emphasize play-action passes to slow the rush on Favre and allow time for routes to develop down the field.
With the goals the Vikings set at the start of the season out of reach, Favre said he merely wants to start the final six games and play well so he can retire on a high note.
Favre said Childress took the fall but the entire team was at fault for Minnesota's disappointing season. Late in the week, Favre said, Childress offered a nice reply to a conciliatory e-mail the Vikings quarterback sent after the coach was fired.
Also inactive Sunday was, as expected, Redskins starting safety LaRon Landry (Achilles) and running back Ryan Torain (hamstring). But cornerback Carlos Rogers returned after missing last week's victory over the Titans with a hamstring injury.
Ed Werder covers the NFL for ESPN. Information from The Associated Press was used in this report.