An aching head, sore muscles, runny nose, as well as lack of energy and an upset stomach are all flu symptoms; however, they can also indicate a food allergy or allergic rhinitis, which can be seasonal or year-round, notes MayoClinic.com. Most people get the flu less than once a year, but if you are experiencing symptoms more frequently, you may have an allergy.
Fever and Aches
Fever caused by influenza has a sudden onset, and can range from 102 to 106 degrees Fahrenheit. Adults generally run a lower temperature than children with the flu. Body aches are another common flu symptom that accompanies fever in the early stages of the illness. Fever and aches from seasonal respiratory allergies like hay fever do not have as clear an onset as fever and aches due to the flu. Fever associated with allergies is also seldom as high as fever associated with the flu.
Nausea and Vomiting
Nausea and vomiting without respiratory symptoms like a cough and runny nose, are not associated with the flu. Some food allergies, like allergies to dairy, wheat and tree nuts, cause a variety of gastrointestinal symptoms. One symptom common of food allergies that is not common to flu is diarrhea. Nausea and vomiting are not symptoms of respiratory allergies, like seasonal allergic rhinitis or hay fever.
A headache is almost guaranteed with the flu, which results from the body's own immune reaction to the influenza virus, dehydration and swelling of nasal and sinus tissues.
Allergic rhinitis also causes headaches, but they are associated with increased mucus production in the nose, ears and sinuses. Food allergies can cause a headache as well, but headaches related to food allergies never occur without related symptoms like gastrointestinal pain and swelling of the face and throat.
Fatigue and Dizziness
People who suffer from all types of allergies almost universally report fatigue and tiredness. With the flu, fatigue and dizziness can sometimes persist long after other flu symptoms have subsided. Dizziness and lightheadedness associated with food allergies can be very serious symptoms, especially if they occur suddenly. If you experience difficulty breathing or your throat is swelling, you should immediately seek medical attention, as this is a sign of anaphylactic shock.