Winry's seen cases of the body reacting badly to automail. Gangrenous infections, night sweats, inflammation. She's a mechanic at heart, but she's also a doctor by trade and she understands the intricacies of the body, it's patterns and peculiarities. She knows that swelling is a natural occurrence in the body's healing process, but the addition of pain and heat that some experience is troubling and she's trained her eye to look at everything critically. Experience has shown that a combination of vigilance and treatment keeps the infection from manifesting into something more sinister. The technical side of her brain can appreciate that.
She's learned through various forms of trial and error to be overly cautious. When a customer complains of some defect in their joints she knows to double check the area where skin meets metal. It's a habit born of perfection maybe, or a secret neurosis she's unwilling to completely admit to. Infections can end in death after all and she doesn't want that left hanging over her head. She's too young for her professional life to be marred by a simple mistake that could have been rectified had she only opened her eyes.
Still, infection is rare and it doesn't take long for her to grow lazy in her overconfidence. Pain can be explained away as a complication of the surgery or poor maintenance on the part of the customer. It hurts, they tell her, but she brushes it off. Some people are just less resistant to pain and she makes this fact known, voice shrill and drowned out by the clanking of machine parts around her. It isn't until someone comes in, eyes glazed with tears threatening to spill, that she reconsiders her quick judgement. Pain is there for a reason, she reminds herself. It alerts us that something's wrong.
Abnormality. Mutation. When a customer can't bend their elbow right because of the swelling she thinks it might be nature's way of telling her that she can't play God. Unconsciously she might fancy herself one, surrounded day and night by nuts and bolts she molds into works of art, of function and practicality that society praises her for. She makes the lame walk and those without arms grasp their loved ones' hands, but she can't ward off the bacteria that creeps into their unhealed wounds. It's a hazard of the business she never anticipated.
A week after his surgery when Ed starts complaining of itching around his shoulder she makes it her job to meticulously scrub the skin each day until it's healed to her satisfaction. In the mornings she oils the automail, checking the range of motion and putting Ed through exercises he is loathe to take part in (his impatience eats away at him these days as Al sits silently at the foot of the bed in a way that unnerves her). Afterwards she fills a bucket with soapy water and carefully sponges the cracks and crevices that line the edges of his automail until he stops flinching from the sting. She wonders sometimes if the extra attention makes him miss his mother, or if it makes Al jealous for his senses, but she doesn't ask. She concentrates on warding off the infection, counting screws at his shoulder blade, ignoring the way her throat catches every time he breathes.