Fred had been by Katie's side ever since the pox marks had shown up and her fever caused her to pass out. She'd been in so much pain that he didn't want to move her or take her to the shack for fear of causing more injury. In short, he was a wreck and he didn't know what to do - other than stay by her side if she needed him.
Neville, Ginny and Parvati were just as ill as Fred was... however, George had shown the signs that Katie was doing. Now both his wife and his twin were both afflicted with severe symptoms and there was no way to help them short of keeping them comfortable and hopefully finding out what in the hell was going on.
When he'd heard Katie's voice, he was pacing around the room and he rushed to her side, sitting beside her on the bed and scratching the back of his neck where his marks were already beginning to fade. "I'm here love." He only touched her hand and tried to stay strong himself. "Would you like me to get you anything? Neville and Ginny are going through the greenhouses to try and find anything that will help."