In truth, Tony hated jogging. If nothing was chasing him, why was he running?
Sam was out of town and Bucky was a far cry medically from being approved for something as exhausting as jogging every day (and he'd gone the past three days) after an injury to his ribs, so Steve had asked Tony if he were interested.
Tony wasn't feeling his best when he woke up that morning, but he'd agreed to go jogging with Steve the night before, so he hauled his ass out of bed anyway.
But Steve had looked hopeful, and he knew that the whole ordeal was less about the running and more about slowly healing a broken friendship, so Tony had said "sure, why not?" when he'd asked.
Turns out, waking up with a splitting headache and a sore throat was "why not."
He'd considered saying something, but when Captain America arrived on his doorstep with a hopeful smile on his face and two water bottles for the jog, he didn't have the heart to say no.
Not even when it got hot before the sun rose.
"Come on, Stark; keep up," Steve called easily, "I'm 100 years old and I'm beating you!"
Tony couldn't quite catch his breath. "Fuck OFF, Rogers," he huffed, "don't remind me."
Steve chuckled and slowed to a stop, handing Tony a bottle of water from his pouch when he finally caught up.
"Drink break," he explained as he thrust the bottle toward him. Tony hesitated--he always did--before grabbing it from his hands.
"Bench," Tony breathed, and the two collapsed into a park bench on the side of the trail.
"Doing okay?" Steve asked, just the slightest bit of concern edging into his tone at Tony's flushed face and wheezing.
"Yeah," Tony replied, "it's just 1000 degrees out here and I didn't drink super serum for breakfast."
Steve bit down a smirk. "I get it," he brushed him off, "we should cut the jog short."
Tony gaped. "This is SHORT?" he asked. "I thought we were done!"
"I've got another three laps in me," Steve subtly bragged, "but like you said, I've got the serum."
Tony allowed Steve to pull him up from the bench by his hands and to steady him when he swayed.
"Sure you're good?" Steve repeated, more concerned this time. Tony'd gone from red to pale upon standing, and his hand had near-imperceptibly twitched toward the arc reactor.
"Just need a sower and some Gatorade," Tony reassured.
By the time they got back to Stark Tower, Happy was waiting outside to drive them to SHIELD for a mission in Wakanda.
In the Quinjet, Tony began to feel worse every passing minute. Without the cool shower and icy drink he'd been anticipating, he was still entirely too hot, and he could feel the heat cycling up from the neck of his shirt and making his face flush.
"Damn, Rogers," Natasha said, reaching into the refrigerator for a water bottle, "I knew you two were jogging buddies today, but you didn't have to kill him."
"For real," Peter agreed. "Where were you running; the sun?"
Steve frowned at Tony, who was leaning back in his seat with his eyes closed, discomfort clear on his face. "He's fine," Steve insisted, taking the water bottle from Nat, "right, Tony?"
It took nudging his hands with the drink to get Tony to open his eyes, and when he did, it was only to glare. Steve rolled his eyes good-naturedly and sat down near the front of the jet to talk to the pilot in typical polite Captain America fashion.
Tony, meanwhile, made no move to open the bottle in his hands.
"Seriously, Stark," Natasha interjected, twisting the cap off without taking the bottle from his hands, "you should drink something. You're not gonna last ten minutes in battle like this."
Tony shook his head. "Nauseated," he replied, "drink it when it passes."
Peter's eyes went wide. "You shouldn't be fighting if you can't keep water down," he fretted. "Are you heatsick or something?"
Tony scowled and took an obstinate sip from the water bottle, grimacing as he felt his stomach turn. "Happy?" he asked, as if he'd proven a point.
Peter was decidedly not happy, but he let it go as Tony closed his eyes and turned toward the window.
Not five minutes later,, Tony bolted upward in his seat.
"Mr. Stark?" Peter called, popping out an earbud and sitting forward worriedy.
Tony said nothing, instead opting to stand and rush to the small bathroom on the Quinjet to throw up the small sip of water he'd managed to get down before.
On the floor of the bathroom, he could feel not only just the oppressive heat from the jog, but the threadiness and speed of his heart rate. His headache, temporarily relieved by the adrenaline of jogging, was now right behind his eyes and throbbing, while his hands were shaky enough that he wasn't even able to lock the door behind him.
Natasha knew that Tony would rather her walk in on him than Peter, so she pushed the kid back and knocked on the closed door of the bathroom.
"Tony?" she tried, dropping the formalities, "are you okay?"
A groan answered her, and she immediately threw open the door.
She cursed in Russian as she dropped to her knees beside him, turning his body so that he was looking at her. He was malleable in her hands, half-conscious, and she cursed again.
"Tony, come on; talk to me," she coaxed. She pressed fingers to his wrist to feel his pulse--fast and weak, but regular--and then to his face, feeling the offending heat radiate from him. "Shit," she muttered, "Rogers knows he's not supposed to let you in the sun that long."
It was true. Dehydration lowered Tony's blood volume, which made his already weak heart have to pump much harder to circulate, and it was easy for him to end up with blood pressure low enough to pass out.
Tony grimaced. "Not his fault," he defended, "I shouldn't've gone at all. Wasn't feelin' so hot this morning."
Great. Now they had a chest infection to worry about preventing.
"You need water," she instructed--the lecture could wait until he was coherent. "Come on." She stood him up on two feet, but everything began to swim in front of his eyes, and what felt like an icy band constricted his head and made him break out in a cold sweat.
"Nat, m'gonna--," he trailed off, collapsing into her. His knees went slack and she was suddenly supporting his full weight until she wasn't.
"Thanks, Peter," she said, and the kid nodded seriously. His face bled concern, near panic. "He'll be okay," Natasha promised, just as she had to Clint's kids time and time again when she sat in the waiting room of a hospital with them and his wife, or when the oldest cornered her before a mission and asked if her dad could get hurt.
Peter shook his head. "I don't--he passed out!" he cried, helping to ease Tony gently into the small bed that they'd made on the jet for Bruce's post-Hulk naps. "He's not okay!"
Natasha smiled sympathetically. "It's just a complication of the arc reactor," she explained. "I'm telling you because I know Tony never will. That thing doesn't just power the suit--it keeps Tony alive. But it's invasive. If he didn't have it, he'd be dead. But at the same time, it comes with its own set of issues."
Peter looked at Tony, wide-eyed and worried. "Mr. Stark never says anything about it being invasive," he admitted quietly. "Does it hurt?" he asked, and it fucking broke Natasha's heart when she realized she didn't know the answer to that.
"He's going to be fine," she reassured instead, "but he's clearly in no shape to fight, and he needs someone to stay here with him."
"I'll do it," Peter offered, "I mean, unless you need me out there."
Natasha shook her head, smiling softly. "No, I think Tony would rather have you here than fighting without him."
After digging through a small box of emergency medical supplies, she finally found a bag of fluids and a needle. She disinfected his arm and stuck it into the vein, breathing a sigh of relief when Tony winced in his sleep. At least he was responsive.
"We've got to get going," she announced, "but if anything changes, let us know. He might be asleep for a while."
Peter nodded and committed her small list of following instructions to memory, then turned back to Tony as she and Steve stepped off the Quinjet.
He spent most of the time that Tony was sleeping building up a lecture for him that Captain America would be proud of.