Wood had thrown 104 pitches through six, and went out for the seventh with a 4-0 lead. The plan was to let the lefty go until someone got on base, and leadoff man Neil Walker hit a changeup to left for a double. Wood was lifted.
"I was ready to go," Wood said of the seventh. "The changeup has been working well, so I just threw it, not really concentrating on getting it down, since it's been working well, and [Walker] did what he's supposed to do."
Pinch-hitter Travis Snider greeted rookie Brian Schlitter with a two-run homer. One out later, Starling Marte singled, and one out later, Andrew McCutchen walked. Schlitter was lifted for James Russell, and Alvarez drove the first pitch from the lefty to center, dropping it on the roof of a batter's eye suite at Wrigley Field.
"It was a slider that I hung and left down the middle of the plate," Russell said, "and that's what he's supposed to do with it. More power to Alvarez for doing his job and taking a bad pitch deep."
"He's come into his own," Snider said of Alvarez. "He's at a total level of comfort. For us, it's fun to sit back and watch him scare pitchers."
Wood had previously struck out nine on Sept. 17, 2012, also against the Pirates. On Thursday, the Cubs lefty gave up four hits and walked three over six-plus innings.
"He did a great job," Cubs manager Rick Renteria said of his lefty. "We sent him back out there [for the seventh], and it was going to be if a guy gets on, he's done."
Wood escaped a jam in the third when the Pirates loaded the bases with one out. The lefty struck out McCutchen on three pitches, then fanned Alvarez to end the threat. Wood was the first left-handed starter the Pirates have faced this year.
"[I had to] bear down," Wood said of the third. "Two good hitters coming up, MVP and Silver Slugger. You've got to give them everything you can, and I was fortunate to throw some good pitches and get them out."
The Cubs lost 43 games last season when they had the lead and finished with 96 losses, and this year, they are 1-4 in one-run games, with all five against the Pirates. Starlin Castro said he feels better about the current team.
"This year, it's closer in every game, more intensity in every game," Castro said. "We've got energy in the dugout that we can tie the game or win the game. It's tough sometimes to lose a game like that. It's tough, but that's the game."
Said Wood: "Last year's behind us, in my opinion. It's just the way it's falling right now. It's baseball and anything can happen in this game."
Emilio Bonifacio reached on a fielding error by Gerrit Cole with two outs in the Chicago third, stole second, and scored as catcher Tony Sanchez's throw sailed past an outstretched Walker and into center field for another error.
Anthony Rizzo and Nate Schierholtz both singled to open the fourth, and one out later, Rizzo scored on Castro's single. An 0-2 pitch to Mike Olt skipped past Tony Sanchez, and Schierholtz scored on the wild pitch for a 3-0 lead. One out later, Welington Castillo singled to left and Castro slid home safely despite a perfect throw home from Marte.
"I don't know if [the umpire] called me safe because the catcher blocked home plate before he got the ball or he called [me] safe because I was safe," Castro said of home-plate umpire Mark Carlson. "I don't know, but I went inside to watch the replay and I looked out."
Castro said the Pirates catcher may have bobbled the ball, and that could be why Carlson called him safe.
"As soon as he called him safe, that was good for us and [we] moved on," Renteria said.
Pirates manager Clint Hurdle did not challenge the call, and said the new obstruction rule was not discussed.
The Cubs threatened in the ninth, loading the bases against Jason Grilli, but the closer induced a groundout from Bonifacio to end the game.
Now, they have to play the Cardinals and Yankees on the road.
"It's good when you play really good teams," Castro said. "We'll see where our team is at. We'll see what we can do and how we can be better. When you play a good team like the Yankees and St. Louis, you have to play 200 percent to win the game."