Loki had never been ill before but being stripped of his powers then abandoned on Midgard soon changed that fact. It was a month, to the day, of being in the care of the Avengers when Loki was struck down with his first illness.
Banner had run many tests to find only the common cold and the only explanation he could find for what ails the trickster was that it was due to Loki not being from Earth, he did not have an immune system that knew Midgardian bacterium and viruses. It took Loki nigh on three weeks of bed rest, and near constant sleep, to get over that cold.
It was during his fourth month on Earth when he experienced the flu. Tony was banned from missions after getting injured on the prior outing so he stayed near Loki as the man required near constant care. The trickster was running a fever and there was nothing anyone could do to cool him down enough.
One night when they were alone in the tower Tony felt completely helpless on how to help the God so he took to begging, something he would never admit to doing. He begged to a deity he didn't believe in, and one that he did. Naturally the God he believed in, the one led in front of him, was the one who answered his prayers, slowly removing the Asgardian facade to reveal his true skin.
The ability to retain his Asgardian features was the one piece of his magic that he was allowed to keep, and this was the only time that he had removed it in front of anyone who was not a Frost Giant. Loki was expecting fear or for anyone who saw him to attack 'the monster' which was why he had never shown anyone previously but as he neared unconsciousness he deemed it necessary.
Tony was somewhat worried when Loki started turning blue but hearing the whimpered plea made him relax (shockingly). "Don't hate me. Please?" Tony watched as the man before him was enveloped in blue skin, culminating with his eyes turning a bright red. The awe in Tony's face relaxed Loki more that the sudden cold that soothed his fever.
Tentatively reaching forward, Tony looked to Loki, silently asking for permission. At the almost unperceivable nod he finished closing the distance between them, grasping Loki's hand within both of his own. He slowly began tracing the muscles and tendons up his arm until he reached the shoulder. Seeing no discontent he then began following the planes of his torso until finally his curiousity had to be voiced.
"Is this a trait of al Asgardians? Does Thor turn blue? I bet that is quite a sight to see."
"No Asgradian can do this."
"But you-"
"Are not Asgardian. Thor told you that I was adopted. I am from a race known as Frost Giants. I am the monster from under the bed in Asgard. A tale only told to children to scare them. Odin fought my father and claimed the Prince of Jötunn as his own. I have only been left with the magic to hide my Jötunn form from others."
"From what you've just said, Odin is the monster, not you, and from what I've seen there is nothing monstrous about the real you. You have no reason to hide your form, at least not when you're with me, you are gorgeous. I mean, umm, well, I meant what I said I suppose." Tony started out calm, yet began to ramble and lose control over his mouth. Something that often happened but rarely did he ever spill his guts so embarrassingly.
Loki watched as Tony said these words and then as he processed what he said and his cheeks began to burn, his heart fluttering like a hummingbird. There was a small smile on Loki's lips as he looked at the man in front of him. The smaller man looked up and caught the red gaze, cheeks burning hotter. Tony finally realised that he still had his hand on Loki, one on his slim wait and the other above his fluttering heart.
Neither of them would ever know who moved first. All they would ever know is that the kiss they shared was the first of many. As per Tony's request he didn't always keep up his glamours around Tony, especially when ill. They soon discovered that the one thing that could help Loki when poorly was allowing his Jötunn side out to cool his fever.
Tony loved Loki no matter which form he took but he especially liked it when he got to see him in all his blue skinned and ruby eyed glory because he had found out, shortly after sharing their first kiss, that Loki had never shown anybody else and he felt privileged for being allowed.