Derek was still feeling off because of Erica and Lucy, made worse by the wrecked howl from Erica just moments before. He wanted to go to her, to do something, though he didn't know what. And he figured at this point she might want to be alone (though that went against his every instinct because pack was always better than the lone wolf act, but he bit it down). He'd be there for her at any second, and she knew that.
For now, he stopped at the store to pick up what Stiles had asked for (feeling only a little guilty in light of everything that he was playing domestic). Erica's words from the other night were sitting at the back of his mind, about him so obviously loving Stiles. Derek was beginning to give in and accept it -- he was going to take care of him while he was sick, for god's sake. But he resolutely decided to ignore that for now.
After getting up on the roof, Derek slid into the room from the open window. Stiles' scent was underlain with that sickly sweet smell of the ill, but it wasn't too bad. Allison had apparently also been by, probably to drop off the soup he noticed.
"I don't think a big cup of sugar is going to make you feel any better," Derek said as he put it and the NyQuil on the table beside the bed, where Stiles was snuggled under the covers. He sat on the mattress, taking off his boots before leaning over to kiss Stiles on the forehead, which was slightly warmer than usual. "How're you doing?"