The virus is spread from the nose and throat of infected persons by droplet secretions to the nose and throat of other people. If susceptible, those people can also become infected and develop symptoms of illness. Anyone can be susceptible. The illness usually resolves after several days, although coughing and weakness can persist for several weeks. If a person has other medical conditions, such as lung or heart disease, they may develop secondary bacterial, or primary viral pneumonia.
Practitioners commonly diagnose influenza based upon clinical symptoms.
Swabs of nasal secretions may be tested by rapid antigen tests, which can determine whether influenza is present. Secretions from the nose and throat may also be tested by culture. Testing of positive cultures can reveal the type of influenza present, A, B, or C, and the strain. This information can be used to determine the contents needed for annual vaccines. Although antibiotics do not work on viruses, a practitioner may prescribe antiviral drugs, which may help lessen the duration and severity of influenza disease.