Flu tests are mostly ordered during flu season (late fall through early spring). Not everyone who has symptoms of influenza requires testing. However, testing is optimal within 48 hours of the onset of symptoms, when a greater amount of the virus is present to be detected and antiviral treatment can have the greatest impact. Testing is recommended in particular for people who are hospitalized, have weakened immune systems, or are otherwise at an increased risk of serious complications. If influenza has already been identified in the community, a health practitioner may order a flu test to confirm the diagnosis and validate possible antiviral treatment. A positive flu test means that the affected person most likely has influenza A or B, and treatment with antiviral medication may be prescribed to minimize symptoms. However, it may not tell the health practitioner which strain of influenza is causing the infection, how severe the symptoms are likely to be, or whether or not a person may experience any secondary complications. A negative influenza test may mean that the person has something other than influenza, that the test is not detecting the influenza strain, or that there is not sufficient virus in the specimen to allow it to be detected. This may be due to either a poor specimen collection or because a person has had the flu for several days and less virus is being shed. New strains of influenza may circulate that diagnostic tests cannot reliably detect.