In the books, I noticed that Alec is described a couple times as having to catch his breath. I don't think any of the others are described like that (although I could be wrong) and I came up with this idea to explain why. Enjoy.
Disclaimer: I don't own TMI.
"Well, that was fun," Magnus panted. He was bent over, hands on his knees as he gasped for breath. Alec was in a similar position next to him. "Not exactly what I had in mind for our romantic date, though," he added. Alec didn't reply, but it didn't worry Magnus. Alec did that sometimes; he just wouldn't reply when Magnus said something sarcastic. Magnus suspected he just didn't know how to reply. It was sweet, Magnus thought, and just so quintessentially Alec.
Alec still didn't say anything, and Magnus was beginning to get worried. He turned to face him and was shocked to see that he was still trying to catch his breath. "Alec?" he asked cautiously. Alec was still struggling for air, apparently unable to get enough. "Alec!" Instantly, Magnus's own slight breathlessness was forgotten. He slid his hands over Alec's body, searching for a wound. It would be on his chest, probably, or on his neck. Magnus searched frantically, but he couldn't find anything.
"I'm fine, Magnus," Alec panted. "I just…" His voice was breathless and finally trailed off as he became unable to gather enough air to speak. Magnus felt his own breath quicken in panic, but he forced himself to control his reaction. That wasn't what Alec needed.
"What do you need?" Magnus asked. Alec just waved him away. "Space?" Alec nodded and Magnus took a step back, still studying him worriedly. "Anything else?"
"Give me a moment," Alec gasped. Magnus dutifully waited for Alec to be ready. It took another minute or so for him to start breathing at least semi-normally again. "I'm sorry," he said weakly. Magnus shook his head.
"You don't have to be. Are you okay?"
"Yeah," Alec replied, straightening. "Can you Portal us home?" The request startled Magnus a bit; Alec normally told Magnus not to strain himself and walked instead. This had to be worse than Magnus had imagined.
"Of course," he replied, though he was feeling a little tired. As long as it wasn't a big Portal and it wasn't up for too long, Magnus should be fine. He would be fairly useless for the rest of the day, but he would be okay.
Magnus scrawled a few runes on the wall behind him, then pressed his palms against it, muttering in the warlock tongue. The Portal roared to life a moment later, immediately beginning to draw from Magnus's reserves of strength. "Go on," he told Alec, hiding the drain he felt. Alec stumbled through the Portal and Magnus followed him, cutting off the magic as soon as he was through. Alec, he saw, had gone straight to the couch and sank down onto it.
"Darling, are you sure you're alright?" Magnus asked as he went over to the couch as well. The second he sat down, Alec leaned over and lay across the other seats, putting his head in Magnus's lap. "Alec, please tell me what's wrong," Magnus begged, hiding his worry over this uncharacteristic behavior the best he could. "Are you hurt? Were you hurt recently and it hasn't healed? Do you need me to help you?"
"It's nothing," Alec reiterated. "It's nothing new."
"Nothing new?" Magnus repeated, puzzled. "What do you mean?"
"It's a long story," Alec warned, shifting a bit.
"We have time," Magnus replied. He wasn't about to let Alec get out of telling him this. Alec sighed. He looked straight ahead, facing away from Magnus.
"When I was eight years old, I was stung by a spider demon."
"You what?" Magnus demanded in a shrill voice. He had been planning on staying silent and letting Alec tell his story, but he couldn't keep from interrupting. "At eight years old?"
"My father took me out to see a hunt," Alec continued, ignoring Magnus's outburst. "I wasn't supposed to ever get close enough to get stung. I was stupid. I didn't pay attention to what my father told me. Anyway, I got stung, and you know what spider demon venom does."
"It's paralytic," Magnus whispered in horror. Alec nodded slightly.
"My father killed the demon and took me back to the Institute. He called for the Silent Brothers. Spider demon venom isn't supposed to be fatal, at least not that quickly, but I was young, so it spread through me much faster than it would have for an adult. By the time the Silent Brothers were able to reverse the effects, my entire body had been paralyzed and the venom had begun to paralyze my lungs." Magnus hissed in a sharp breath. If the venom had gotten that far, it had gotten very close to killing Alec. "The effects of the venom wore off fairly quickly, but they did cause some long term effects." Alec closed his eyes. If Magnus didn't know better, he would say he was ashamed of the story, but that was ridiculous. He had nothing to be ashamed of. "After a lot of training and…oh, what do mundanes call it, when they do special exercises after an injury? Physical therapy?" Magnus nodded, then realized Alec couldn't see him and voiced the answer softly. "Well, anyway, after that, I was almost back to normal, but because the paralysis had occurred when I was so young, my muscles never fully recovered. It meant I would never be quite as strong as I would have been if I hadn't gotten stung." Alec paused, but Magnus knew the story wasn't over. After all, he hadn't addressed what had led him to tell the story in the first place: the issue with his breathing.
"Because the paralysis spread to my lungs, my lungs have never been quite as strong as a normal Shadowhunter's," Alec continued softly. "It's nothing serious, really. I just get out of breath faster than most other Shadowhunters would. I need more stamina runes than Jace or Izzy because I burn through them faster. Normally, I'm fine with a rune or two, but since we were out on a date today, I wasn't expecting to need to run as much as we did. I didn't apply any stamina runes. That's why I was so out of breath. So that's the story." Alec's voice trailed off awkwardly at the end. Magnus sighed deeply and began carding his fingers through Alec's hair.
"Oh, my darling Alec," he murmured. "You're so strong."
"What do you mean?" Alec demanded, pulling away from Magnus to look up at him. "I just told you the story of why I'm not strong. I can never be as strong as Jace or Izzy or any other Shadowhunter because I made a stupid mistake when I was a stupid little kid."
"It wasn't your fault," Magnus contradicted. "You were eight years old."
"I shouldn't have gotten close enough for the demon to be able to sting me," Alec protested.
"You shouldn't have been there, period," Magnus said firmly. "You were a child. You hadn't even gotten your first Marks yet! If anyone should be blamed, it's your idiotic father for taking a child out on a hunt with him."
"He told me not to get too close," Alec objected. "But after he killed the first spider demon, another one came out of no where and attacked him. I tried to attack it, which was a dumb idea, since I only had a dagger with me. That's how I got stung. My dad could have handled it. I only made it worse."
"Sweet, precious Alec," Magnus sighed. "You tried to save your father's life and you're calling it stupid?"
"He didn't need me. He would have been fine."
"You were eight years old. You were scared, I'm sure. And you still attacked a spider demon that was probably at least twice your size to try and save your father. That's one of the bravest things I've ever heard."
"That's why I'm afraid of spiders," Alec added. "It's ridiculous, I know, but I can't shake it."
"It is not ridiculous," Magnus protested. "God, Alec, you…" Unable to even put his emotions in words, Magnus pressed a kiss to Alec's lips. He poured all of his passion into it, kissing Alec with everything he had. When the kiss ended, Magnus pulled back a little bit so he could look Alec in the eyes.
"You are not weak," he told him firmly. "You have this disability, but you've never let it hinder you. You're just as good a Shadowhunter as Jace or Izzy, despite everything. You are the strongest person I've ever met, Alec Lightwood, and I love you."
"I love you too," Alec whispered, leaning his head on Magnus's shoulder. He smiled. "You take my breath away."
And Magnus couldn't help but smile at the terrible pun.