Influenza A and B H1N1 H3N2 Hemagglutinin-nucleoprotein recombinant proteins, peptides and antibodies detect a virus commonly known as "the flu". Influenza is an infectious disease caused by an influenza virus. Symptoms can be mild to severe. The most common symptoms include a high fever, runny nose, sore throat, muscle pains, headache, coughing, and feeling tired. These symptoms typically begin two days after exposure to the virus and most last less than a week. The cough, however, may last for more than two weeks. In children, there may be nausea and vomiting, but these are not common in adults.