OAKLAND, Calif. -- Five years ago, veteran big leaguer Nick Hundley visited Children's Hospital of Wisconsin and met a boy named Zach Sprader. Hundley, then with the San Diego Padres, sought to give the young baseball fan a lift during his fight against leukemia. Little did Hundley know a lasting friendship with a family would come of it.

A couple of weeks after his charitable visit with Athletes for Hope, Hundley was traded to Baltimore. Two weeks after that, he returned to Milwaukee with the Orioles for a series against the Brewers, and that gave him a chance to visit the teen again. Hundley vowed to stay in touch with the family, which saw Hundley play in Baltimore later that season.

"I took him around, hung out in the clubhouse,'' Hundley said. "It was awesome.''

"The two of them became fast friends,'' Zach's mother, Lisa Kukec, recalled Monday at the Oakland Coliseum. "We stayed connected, and when we were on Zach's Make-A-Wish to the East Coast, we were able to see Nick at Camden Yards. Nick and Zach stayed connected, and we would connect with him when his team came to Milwaukee to play.''

Zach died just more than a year after that initial meeting, in October 2015 at age 14.

Still, Hundley kept his word with the grateful family -- Lisa, Zach's father, Chris, and younger sister, Ariana, who wear matching, orange "Team Zach'' shirts with his jersey No. 24 on back for the former shortstop and catcher. On a visit to Oakland with hopes of seeing Hundley -- now a catcher with the A's -- they were delighted to see No. 24 painted on the grass for Hall of Famer Rickey Henderson Field, just one more fitting moment of this journey for Zach.

Before he died, Zach visited seven ballparks. The family is on a quest to see all 30 major league venues in his honor. Ariana, who is 12 and will be a seventh-grader, carries a leather-bound passport booklet that she gets stamped at each stop to keep track of the stadiums.

On Sunday, the family crossed No. 12 from the list, watching the Brewers play the Giants in in San Francisco. Monday at the Coliseum marked No. 13, and they stood behind the cage for A's and Orioles batting practice thanks to field passes provided by Hundley. They will be at Petco Park in San Diego on Wednesday, then Dodger Stadium on Friday.

"The Angels were out of town, so we'll have to come back,'' Chris Kukec said.

On Monday, A's president Dave Kaval stopped to say hello and promised to help with any hiccups acquiring tickets during their visits. Just so happens that Kaval hit all 30 ballparks in 38 days after graduating from Stanford, driving 1,500 miles.

Hundley wasn't around for Monday's visit. He left the Coliseum early after getting treatment as he rehabs from a back injury.

He keeps in regular touch with the family, though. He had no idea that spring day in 2014 he would make a lasting impact and friendships. Hundley and Zach took pictures together, and the teen signed his baseball card for the big leaguer.

"It's one of those things where you're going into a really hard situation and you're just trying to take their mind off it for two minutes or 10 minutes,'' said the 35-year-old Hundley, in his first season with the A's and 12th in the majors. "Or they get to know you and think about the game and think about something other than being sick and being in the hospital all the time. Zach was a big baseball fan, and I was really fortunate to be there to meet him. Knowing him, I feel like I get more out of it than he did just seeing his battle. ... Just a lot of good memories."

The Kukecs live outside Milwaukee, so as Lisa explains, "We're essentially Brewers fans, but we're Nick Hundley fans.'' Every time he's in town, Hundley visits Lisa at Children's Hospital, where she now works. "Just one of those cool things,'' Hundley said.

Oh, the family feels the same way, and then some.

"Since Zach passed away, we have continued to keep in touch with Nick and continue to complete Zach's wish of touring every stadium,'' Lisa said. "Nick has a heart of gold and truly touched Zach's life, as well as ours.''