Jason “Sick” Day has taken a turn for the weird. Now, rather than simply suffering the injuries himself, it appears he has become a carrier.

No one knew that backaches were contagious until this week at the Tour Championship, when Day’s coach and career-long caddie Col Swatton pulled out in mid-round with one. That occurred shortly after Day was forced to withdraw from last week’s BMW Championship with the same affliction.

Considering the string of physical issues that have stood between Day and his appointment with greatness — thumb, viral infection, now back — fans at East Lake are advised to approach him with caution. Make sure you’re taped up and have all your shots.

The good news is that such a season of trial has left Day nothing if not resilient. He pulled his mental coach, Jason Goldsmith, out of the gallery Thursday to carry the bag, started stepping off his own yardage and positively flourished. His 3-under 67 on Friday left him at 6 under through two rounds of the Tour Championship, just two back of leader Billy Horschel.

And that face he’s making now — it’s not a grimace; he’s just bearing his teeth.

“Playing injured — or playing with niggling injuries, it’s definitely not the spot you want to be in. But it’s something where you just have to suck it up and play and try to fight for the win.

“And that’s really what it boils down to: Do I want it enough? I’ve got to want it more than the guy who’s leading now and the guys who are chasing.”

The guy who is leading is going to be tough to reel in, so long as he keeps feeding his red-number habit (66-66, 8 under).

There is a beautiful symmetry to Horschel’s designs for this week. “Like I said (Thursday), I’ll take three more 66s and see where that puts me. And I’ll say now, I’ll take two more 66s and see where that puts me on Sunday.”

And among those chasing are players completely devoid of sympathy for Day’s afflictions. The world’s No. 1, Rory McIlroy, was the most notable to state his intentions with the day’s low round of 65. He got his groove back in a serious way, registering 64 percent in fairways found and a day’s best 77.8 percent of greens in regulation.

And he batted 1.000 in the little known stat: Par Saves From Galley Garments. Nothing seemed to shake McIlroy, not even the one-in-a-billion tee shot that caromed off a tree limb into the pocket of a fan. He took his drop and parred the 14th hole like he was a mailman delivering another letter.

In all, Day is one of seven players within four shots of the lead, two of those — Chris Kirk and McIlroy — are, along with Horschel, positioned to win the $10 million FedEx Cup bonus by winning this tournament.

Day needs some help for that big prize. But if anyone deserves a gift basket of money, along with a heartfelt get-well card, it’s he.

He was being measured for a big year by those who know his talent. But Day wasn’t quite right at the Masters and had to pull out of the Players Championship with more thumb problems. Cancelling a handful of other tournaments, withdrawing from two others in the past six weeks, he never was able to get any traction.

Day said he is still not 100 percent, that the back and the thumb still register small complaints. He’s only 26, but conversations of his year always seem to tilt toward what’s aching now, like a visit with your grandparents down in Florida.

“Yeah, it has been a frustrating year,” he said. “The body has not performed the way it’s supposed to. … I’ve played pretty solid golf, a bunch of top 10s (five), but I’m always expecting more of myself.”

“I think in the offseason I’ve got to get a handle on my injuries and move forward and hopefully I don’t have any problems in the future.”

If Day is upright and functional, he’s dangerous. And this caddie thing hasn’t ruffled him at all; if anything perhaps it has sharpened his focus on the course. Getting his own yardages, clubbing himself, “reminds me of the old junior and amateur days,” he said with a smile.

And by season’s end he has to be running out of new ailments to slow him, doesn’t he? So, barring an outbreak of shingles, gout or scurvy, Day just may make it through the weekend in fine form.