I'm sick right now with a sore throat, mild fever, and muscle aches (either a virus or a combination of allergies and fibro flare from the weather changes).
I'm eating regular lower carb foods as my meals (today hubby made bacon and scrambled eggs for breakfast), but my symptom relief foods are so low-carb and low-calorie that they're not really "foods" per se. They're essentially water and flavored water (either flavored with spices or with artificial sweeteners). Low-carb simple broths (made with a low-carb bouillon or with canned chicken broth). If hubby's in the mood, he'll turn broth into real food by adding add meat and veggies, but often it's just hot broth with garlic (anti-microbial properties), and spices including hot peppers (because capsaicin, the hot in hot peppers is an amazing decongestant and mucus thinner).
If you don't mind artificial sweeteners, sugar free popsicles are nice for a sore throat. Just check that the sugar substitutes are not a sugar alcohols (which always end in ol). Sugar alcohols can have a laxative effect, sometimes to the point of diarrhea, so that's not a symptom you want to add when you're already ill.
I'll also drink plain water, or Crystal Light lemonade or other drinks from mixes (you have to check the drink mixes, the orange drinks especially, because many often contain caffeine). If I'm really congested I add cayenne pepper to the lemonade (which I wouldn't recommend unless you're a serious chili-head). I love hot peppers and I can't say that I love cayenne lemonade, but when I read about it in a chili-magazine I thought it was worth trying. The magazine recommend drinking it hot, which I can't stand at all. So I drink it cold. If you like hot peppers it's tolerable, but I find the symptom relief is worth the ickiness factor.
If you don't like hot peppers, the mucinex works fine, but if you like spicy food, adding a little cayenne or other hot peppers to foods and beverages can work wonders.
Mashed potatoes is one of the foods my mother would make when we were sick with a sore throat, so I often crave them, but I like mashed cauliflower just as well.