To understand why Harrison Smith seems to be at his happiest when crashing into ballcarriers, why the Vikings safety throws his body around on a football field every Sunday, you need to give his mother a call.

Susan Smith will gladly tell you story after story about young Harrison, the daredevil of a family with three boys and one girl, though she hopes he won't get too embarrassed that she is sharing them with you.

There was the time at Natalie's fifth birthday party when 2-year-old Harrison roller-skated full-speed into the wall, then dusted himself off and did it again and again, a smile on his face the whole time.

Or the time when Susan taught Harrison to ride a bike, so his older brother couldn't pedal way from him anymore. And as soon as she let go of the handlebars, he sped down the street, over a curb and into the neighbor's bushes, before springing up and explaining, "Mom, I had to stop somehow!"

She remembers when Harrison chose soccer over peewee football one year and was miserable about it. Susan and Harrison's father, Steven, looked in the yard one day and saw Harrison in full football gear, spinning, juking and tackling trees.

"He never was afraid of contact, shockingly," Susan says with a laugh. "Never."

That won't seem shocking to anyone who has watched Smith during his third season with the Vikings. The 25-year-old has thrived in a Swiss Army knife-type role for new head coach Mike Zimmer, who is putting Smith in position to slam into pretty much every player allowed to touch the football. Through six weeks, Smith has generated Pro Bowl buzz, intercepting three passes while becoming one of the leaders of an improved Vikings defense.

"The thing I like the most about Harrison is he's a good competitor," Zimmer recently said. "He's got a lot of toughness in him, a lot of grit."

Smith showed those qualities before he was old enough to strap on a helmet.

Early scare

When Smith was 2, he started burning up with a fever. His parents, thinking it was just the flu, took him to get checked out. It was much more serious.

His red blood cell count was low, and doctors narrowed it to two possibilities. Thankfully, Smith did not have leukemia, but hemolytic uremic syndrome was also a life-threatening condition, especially at his age.

The disease causes red blood cells to destruct prematurely, and they can clog the filtering system in the kidneys. Doctors feared he needed a kidney transplant, but Smith fought it off. After a week at the hospital, he was healthy enough to return home. It had no long-term impact on his health, and he was so young that he remembers nothing about the ordeal.

"He overcame it. It was very serious," Susan said. "There were moments where we didn't know if he would make it. It's amazing to me that such a strong little boy came out of that."

Football wins out

The family moved from Georgia to Tennessee when Smith was 5. He played just about every sport there was to offer — baseball, basketball, soccer, karate, track, football. He loved "The Mighty Ducks," so once he laced up the skates and tiptoed out onto an ice rink.

"It was so hard, probably the hardest sport I've ever played," Smith said, shaking his head.

His competitive nature was fueled in part by trying to keep up with his older brother, Garrett, now 31. Susan said that Harrison would "get fire in his eyes" trying to keep up with the big boys.

When Harrison got older, he rode dirt bikes, skateboarded and loved chasing thrills out on the lake, whether he was riding a Jet Ski or being dragged on a tube or a wakeboard behind the family's boat.

He still heads home in the offseason to spend time on the water, play fetch with the family's two chocolate Labs and shoot hoops in the back yard with Garrett and Stuart, 19, who plays college football at Furman.

But football has always been his first love, dating to kindergarten, when his teacher asked him for a picture of what he wanted to be as a grown-up and he drew two: Harrison the doctor and Harrison the pro football player.

South Bend star

Smith starred as a running back and defensive back at Knoxville Catholic High School. Steven, a plastic surgeon, would stay up late making VHS highlight tapes to mail to colleges. The recruiting letters started to fill the mailbox. One of the first was from Tennessee, which was right there in Knoxville, but Harrison had his heart set on Notre Dame.

Smith redshirted his first year and was given a specialized linebacker role the following season. With so many of the Fighting Irish's opponents using spread offenses, Notre Dame countered by using Smith at a hybrid position where he did a little bit of everything.

"There were a lot of similarities [to his current role with the Vikings]," he said. "And even if you're playing safety, you are going to do some things that linebackers do. So that was good for me. I blitzed a lot."

Smith moved back to safety as a redshirt sophomore and became the only player in Fighting Irish history to record more than 200 tackles, 15 tackles for a loss and 15 pass breakups during his career.

The Vikings traded back into the first round to draft him in 2012, and he started right away for a team that went 10-6 and made the playoffs. Smith made 104 tackles as a rookie and took two of his three interceptions back for touchdowns.

The following season, he missed eight games with a toe injury. The Vikings won just five games and coach Leslie Frazier was fired afterward. The losing took a toll on Smith.

"I don't want to say that you necessarily get down on yourself. But you only play to win," he said. "Everybody gets paid. That's awesome. But after you lose so many games, you're like, 'I don't really care how much I get paid right now. I just want to win.' "

Versatile Viking

The Zimmer era started with two wins and four double-digit losses, but Smith's play in the secondary is one of the main reasons the Vikings are allowing 65.5 fewer yards per game and 6.2 fewer points than they did a season ago, when they had one of the NFL's worst defenses.

Smith, at 6-2 and 214 pounds, is listed as a free safety, but he does more than just patrol the deep middle. One snap he might line up in the box, then dart into the backfield to drag down a running back. On the next, he might cover a tight end or drop into one of Zimmer's zone looks. And on third downs, he can usually be found at the line of scrimmage, threatening to barrel into the backfield on a blitz.

"It's a lot of fun because you can get in on a lot of different types of plays," Smith said.

Smith has done just that. He is tied for the NFL lead with three interceptions, ranks fourth among Vikings with 31 tackles and has disrupted quarterbacks as a blitzer.

Defensive backs coach Jerry Gray doesn't think there is anything that Smith can't do, besides maybe putting his hand in the dirt as a defensive tackle.

"Right now, I haven't seen it. The good thing is that we don't need him to play D-line," Gray joked.

Paying it forward

Tuesday night, Smith showed his softer side by hosting about two dozen children from Big Brothers Big Sisters at Winter Park for his inaugural "Kickball Classic." When he pitched, he slowly spun lefthanded rollers to the grade-schoolers. When at the plate, he waved the children back, then sneaked in a bunt.

Before the event was over, Smith spoke about the importance of positive role models and encouraged the children to chase their dreams, just as his family encouraged him to do.

"They're my backbone," he said later. "My parents obviously set a great example for us and did everything they could to help us with what we wanted to do."

For Smith, that has always been crashing into opposing ballcarriers as if they were his neighbor's bushes.

"He's a very kind and empathetic person, and he always was even as a child," Susan said. "But when it comes to competition, he has always been fierce."