Meredith Vieira to sub for Bob Costas on NBC

NBC is turning to Meredith Vieira on Friday and probably at least one other day to substitute for Bob Costas, who continues to be held off the network's prime-time Olympics broadcast because of an eye infection.

Costas is feeling better but his participation is a day-to-day decision, said Jim Bell, executive producer of NBC's Olympics competition, on Friday. If Costas' illness lingers into next week, the network will consider shutting him down for the rest of the games, he said. Matt Lauer subbed for Costas for three nights starting Tuesday.

For someone with an infection in both eyes and sensitive to light, a TV studio is a lousy place to be, Bell said.

"He's frustrated, of course, to have this happen but appreciative of the support he's been getting both internally and externally," he said. "His spirits, relatively speaking and all things considered, are pretty good."

With Lauer continuing to anchor the "Today" show every day and making himself available for stunts like a luge ride on Friday's show, Bell said he needed a break. Vieira was an attractive alternative because after co-hosting the opening ceremony, she's largely been on hand to do features. Picking someone else would disrupt another part of NBC's schedule, he said.

Bell was the top producer of "Today" when Lauer and Vieira were the co-anchors.

Vieira makes some NBC history as the first woman to host a prime-time Olympics telecast for the network. But it's hardly a stretch: a key factor in the success of a prime-time Olympics telecast is getting women, not just male sports fans, invested in the games.

She said that she'll anchor the telecast for "as many days as they ask me to, but I hope Bob is back tomorrow."