Can Alex Smith keep this up?

Alex Smith is having a great year so far for the 49ers. But does Jim Harbaugh deserve all the credit? AP Photo/Paul Sakuma

The San Francisco 49ers are 4-1. This is a surprise. Quarterback Alex Smith is playing well. This may be an even bigger surprise.

For the most part, the 49ers have gotten off to their hot start because of defense and special teams. Both units rank second in the league according to the Football Outsiders' DVOA (defense-adjusted value over average) ratings. But the 49ers are also showing a strong passing game, led by Smith, a previously undistinguished quarterback.

Smith is completing 65.9 percent of passes with seven touchdowns and one pick, plus a career-high 7.7 yards per attempt. According to Football Outsiders stats, Smith has been the 10th-best quarterback in the league on a per-play basis this season.

The question is obvious: Can Smith possibly play this well going forward? And how rare is this kind of improvement from a quarterback who struggled so much in his first six seasons?