Titania is the strongest woman in Fairy Tail, not just in terms of physical strength but also in regard to her immune system. She has no allergies, her injuries tend to heal fast and she has never suffered from a cold in her life.
Titania doesn't get sick.
"She has the flu." Porlyusica retrieved a bottle with some concoction and a small jar of pills from her medical bag, handing them to Lucy. "Give her one of these every eight hours and an analgesic for the headaches."
"I don't have a headache," Erza argued between sniffs, trying unsuccessfully to recover the ability to breathe from both nostrils.
"You will." The old woman then smacked the back of her head to chastise her, "And stop that! You need to expel all the snot in your nose."
She turned back to the blonde to keep explaining while the redhead mumbled something indignantly. As the woman finally left, Erza started to get off the bed but Lucy grabbed her shoulders and pushed her back down.
"Where do you think you're going?"
"I have things to do at the guild."
"Nuh-uh, she said you need to rest."
"I'm fine! I just stood too long under the sun!"
"Babe, don't be so stubborn."
"I'm not sick! I never get sick, there must be dust in the air."
"Erza, is the middle of winter! Is normal for people to get sick!"
"I'm not–"
"Yes, you are! And I won't let you leave this house."
The redhead pouted and crossed her arms like a child. Lucy left to retrieve a glass of water so Erza could take her medicine. When she returned, the bed was empty. She caught the other woman right at the entrance door.
She grabbed the knight by the arm, but even in such conditions, she was way stronger than Lucy. The redhead opened the door and there they found Juvia, standing with grocery bags in her arms.
"What's going on?"
"Juvia, help me take Erza back inside!"
The blue-haired girl left the bags aside and did as she was told. The strength of both mages combined served to tire her, and they finally managed to drag her back into bed.
Later that day, the two girls were trying to convince their girlfriend to take the medicine. Erza had no problem with the pills, but she absolutely refused to drink the red syrup that was supposed to help with her throat and the snot.
"It smells weird!"
The celestial mage sighed for the hundredth time that day. "You just have to drink a little," her response was a groan.
"Come on, darling," Juvia said in her softest, sweetest voice while batting her eyelashes. "If you take your medicine, I promise to make a cake when you get better."
The proposition made her perk up. Erza downed a shot of syrup in one go, making a face at the end because of the taste. Then, after a moment of consideration, she snatched the bottle from Lucy, chugging down the rest.
"Erza, no!"
"But if I drink it all now, I'll get better by tomorrow." The blonde pinched the bridge of her nose. Erza had managed to drink almost half the bottle, it only took some minutes for her to start feeling doozy and fall asleep.
The redhead slept for what was left of the day and woke up around midnight, feeling her whole body aching, The sickness had finally settled in completely. She shifted in her bed for almost two hours before slowly standing up and dragging her feet to the other room, where Lucy and Juvia would be sleeping while she recovered.
She debated whether to wake them up when the blue-haired woman turned in bed, yelling as she caught sight of her in the dark. The celestial mage woke up, startled, and turned on a bedside lamp.
"Erza? What's wrong?"
"I feel like crap." She sat at the edge of the bed and looked at them with an exaggerated pout. "I don't want to sleep alone." The water mage immediately went to hug her, but Lucy kept her distance.
"I'm sorry, but no. You could pass the flu on to-." She tried to be the voice of reason, however, Erza made a pitiful whine while the other girl imitated her pout and the blonde was just too tired. "...Fine, but you have to face away from us."
They laid down so Erza and Lucy would be at each end with Juvia in the middle, holding her sick girlfriend. The redhead was able to focus on them instead of her body's aching and fall asleep once again.