There is a saying in the industry that NFL coaches don't get sick. Coughs turn to chest-rattling hacks; virulent stomach flus are handled with fluids and very little discretion, and the season churns on, just like a football coach. They don't do doctors' offices. They live by different clocks, which run somewhere from 5 a.m. to midnight.

When Brian Heimerdinger called his dad last month, he knew something was off. Big Mike Heimerdinger, offensive coordinator for the Tennessee Titans, toughest man he knows, was in the hospital getting tests. Brian asked if he was OK; Mike quipped something sarcastic like, "Don't worry, you're not getting the inheritance yet." And both men, football men, went back to work.

Three days later, Mike Heimerdinger called with news that rocked the Titans: He had cancer, and would immediately start undergoing chemotherapy treatments. Brian, an intern scout with the Houston Texans, hopped on a plane and rushed home to see his father.

They'd talk about the things that somehow get lost in the road trips and 15-hour workdays. About love, life, fear and mortality. Surely, they'd talk about slowing down. When Brian arrived home in Nashville, his dad wasn't there.

He was at work.

"I walked into his office and gave him a big hug," Brian said. "He said, 'Hey, it wasn't my plan.'

"But you just roll with it."

On Tuesday, after much hesitation, Mike Heimerdinger said he'd do this interview under one condition. He didn't want any of his current players interviewed. "He doesn't want them to have to deal with his issues," said Robbie Bohren, the Titans' director of media relations.

He doesn't want any attention. But he made a couple of million hearts melt last weekend when the Titans released a short video clip after the team's 31-17 win against Houston on Sunday. It showed coach Jeff Fisher giving 'Dinger the game ball, and Heimerdinger being one part embarrassed and five parts deeply touched.

The team gave him a warm round of applause, and Fisher embraced his friend. As he emerged from the tunnel late Sunday, he was clutching onto his briefcase and the football, still wearing a huge smile.

Heimerdinger had a stocking cap over his freshly shaved head, his hair a casualty of the chemo. Last week, when he was walking through the hallways of the Titans' facility, another clump of hair fell out and into his hands. So he asked the team trainer, who happens to be bald and owns a pair of clippers, to just shave the whole damn thing off.

"When it first came out, I think people thought I was dying and on my deathbed," Heimerdinger said. "I'm here working, and will continue to work.

"I've had people say, 'Oh, this is a great thing you're doing.' [But] that's the first thing the nurses tell you, that you should go to work, you should stay upbeat if you're going to beat this thing."

In the month since he was diagnosed, Heimerdinger, 58, has not publicly said what kind of cancer he is battling. On Wednesday, he called it a "small-cell, fast-moving cancer." He started chemotherapy right after his diagnosis, with his wife Kathie beside him for the first treatment, along with Brian. His priest sat with him for the next chemo session, and then Heimerdinger's parents came along on the third day. Heimerdinger will undergo his second round next week.

Being sick, he said, has made him more cognizant of life on the outside, away from the insular world of an NFL coach. He sees the guy at the hospital who drove nine hours for treatment, then headed nine hours home. Or the people who sit in the banks of chairs around him, fighting this disease anonymously.

"I've gotten great support," Heimerdinger said. "I mean, it's unbelievable, the letters and the calls and the notes. The other day I saw a buddy of mine I haven't seen in a couple of years that I used to coach with at Florida. He hates flying and drove in from Providence, R.I., to see me. He wanted to make sure I was OK."

Before the tests and diagnosis, Heimerdinger struggled for a good part of the season with back pain so excruciating that he couldn't sleep. So he'd come to the office at 4 a.m., then slam caffeine to keep going through the day.

He can't keep that breakneck pace now. He does 12-hour days instead of 15. He comes in an hour later for work, and takes an occasional nap during the day in a recliner the team purchased for him. He is more careful with his diet. He has a port in his chest, a little reminder, he joked, to calm down when things aren't going the Titans' way.

He insists he feels great, but his colleagues and family wonder if it's 'Dinger bravado. Sometimes, the assistant coaches will text Brian and exchange notes on how they think he's feeling. But they'll never get a completely honest answer from Mike.

"We have our ways of gathering information," said Titans quarterbacks coach Dowell Loggains. "You ask, 'Hey, Coach, how are you feeling?' and he says, 'I'm good. Not bad.'

"When it first happened, I think the initial reaction was, 'I can't believe he's doing this; I can't believe he's coming to work.' You can see, when he starts to get tired, he just perseveres."

Tough, grumpy and good

Let's not sugarcoat this. The clip is so heartwarming because it's in the backdrop of a violent and unforgiving sport. Mike Heimerdinger will admit it, he's not a warm and fuzzy guy. When Brian was a little boy, his friends used to be afraid of his dad. Heimerdinger's colleagues call him gruff and intense, but say it as if it's the ultimate compliment.

"The consensus on Mike Heimerdinger," Loggains said, "is that he's a tough, grumpy guy who really is a good person. Who people care a lot about. He has an infectious personality. When you're around him, once you break that exterior, you really see what he's about, what kind of heart he has."

Mike Heimerdinger says he is being treated for a "small-cell, fast-moving" type of cancer. Brett Davi/US Presswire

The Heimerdingers estimate that they've lived in at least 16 houses during Mike's 36 years of coaching. Mike, his son said, coaches with a chip on his shoulder, because that's what competitive people do. He is blunt and sarcastic, a perfectionist who's careful with compliments.

He has been known to explode occasionally, not because he yearns for the attention of the cameras, but because he's a prideful man who hurts when things don't work out.

He has spent roughly half of his NFL career with Fisher, helping mentor the late Steve McNair during the franchise's successful run in the early 2000s, then returning in 2008 after jobs with the New York Jets and Denver Broncos.

Heimerdinger is known for having a sharp eye for personnel, and Fisher says he's never been around a better game planner and playcaller. The 2010 season was supposed to be a memorable one in Nashville. The Titans had won eight of their last 10 games in '09, and had the league's leading rusher in Chris Johnson and an electrifying quarterback in Vince Young.

But by November, Young was injured and on the outs with Fisher, and the Titans were in the middle of a six-game losing skid while Heimerdinger was making trips to the hospital.

"Sometimes," Fisher said, "you do your best work under the most difficult conditions or circumstances. Sometimes, it doesn't convert into wins. But still, coaches know when the work's getting done."

In an effort to preserve his strength, the Titans moved Heimerdinger from the sideline to the booth on game days. Heimerdinger jokes that it's more comfortable up there now that the weather has gotten cold, and that he planned it this way. But it's clear he doesn't like the obscured view from above.

It hasn't affected his play calling. On the first drive of the game Sunday, Fisher decided to go for it on fourth-and-7½, and Heimerdinger assured him he had the perfect play. He dialed up a deep shot, which resulted in a 39-yard pass to Damian Williams. Some wondered if it was a sign that 'Dinger was going for broke. But Heimerdinger says he's never been afraid to take risks.

But there are a lot of sides that people don't see, Brian said. He's a tough guy who drinks beer on the porch and sings country music, he's a doting father who missed just two of his son's high school football games in four years. He's the competitor who still beats his son in golf, and still playfully snatches away the money that they wagered.

Brian talked about his dad deep into the night on Wednesday, then texted a few minutes after the phone call ended.

"I know him as the guy who goes to church," Brian said in the text, "and will go out of his way to help people. I know him as the guy sitting on the couch quoting movies. So it may be that he is known for how he coaches, or what he is like on the field. But that's just a small part of who he is.

"He's an unbelievable person, and I hope to one day live up to his name."

'You're not going to break the guy'

Who knows why the cancer has come? Was it the exhaustive work days, the poor diets, the constant stress? Heimerdinger doesn't think much about that. He focuses on the success stories of other coaches who have beaten the disease. He refuses to sit around and mope.

Former colleague Herm Edwards isn't surprised that Heimerdinger is still coaching. For one long season in 2005, he got an up-close look at his persistence. Heimerdinger was offensive coordinator for the Jets. He lost four quarterbacks. He was in a new town while rumors swirled that Edwards was leaving for a job in Kansas City.

Heimerdinger, with QB Vince Young last season, is known for having a sharp eye for personnel. Kyle Terada/US Presswire

"He never blinked," Edwards said. "It was kind of like, 'Don't worry about it. We'll move the ball.'

"He's an old-time football coach. When he says he's going to do something, he's going to do it. I mean, you're not going to break the guy. He's got tremendous will. He doesn't want a pity party. Coaches are into fixing things. That's what they do. Coaches don't sit there and dwell on the problem. They find the solution."

He is tired and drained in a 6-8 season that would suck the fight out of almost any coach. He is in the office by 6:30 every morning because this is the schedule and the life he knows. It doesn't matter that the Titans have just a sliver of a chance to make the playoffs. A competitor finishes out the season.

"He wants to see this thing through," said former Titans tight end Frank Wycheck. "Finish what you start. He's been an amazing example.

"You're in there every day, and during the season, you're around those guys more than your family. It definitely hits home, and everybody has a stake in it. I would think everybody in that room is going to do a little extra for Mike."

Heimerdinger brushes off the notion that he could be an inspiration, and steers completely away from the mushy stuff. But friends have seen it, how cancer has changed the big man just a little. He stops to hug people more now. He walks into the stadium a little slower, taking everything in.

Then, it's back to work, which, in the NFL, never really changes. On Christmas, he'll board the team plane and head to Kansas City, drawing up a victory, hoping this season doesn't end. No, cancer hasn't made him re-evaluate his holiday plans. No, he won't be holding hands with his family while a spiral ham bakes in the oven.

It's late December, and Heimerdinger has a lot of living to do.

"My mom had to call me this year to remind me it was my birthday," he said. "They're just days. Christmas and Thanksgiving You play all the time. I've got a job to do, and as long as I'm healthy, I'm going to continue to do this job."

Elizabeth Merrill is a senior writer for ESPN.com. She can be reached at merrill2323@hotmail.com.