Flu in Adults: Flu (influenza) is an acute infection of the nose and throat airways. This infection may spread to the lungs. Flu is the most frequent cause of acute respiratory infection in people of all ages. Symptoms of the flu include fever, severe aches and pains, headache, dry cough, sore throat, and at times vomiting and nausea. Treatment for the flu include home remedies such as OTC pain relievers, rest, and hydration. Medical treatment for the flu include antiviral medication and antibiotics.