Symptoms range from mild to severe and often include fever, runny nose, sore throat, muscle pain, headache, coughing, and fatigue.

Other complications include acute respiratory distress syndrome, meningitis, encephalitis, and worsening of pre-existing health problems such as asthma and cardiovascular disease.

Influenza infection is diagnosed with laboratory methods such as antibody or antigen tests and a polymerase chain reaction (PCR) to identify viral nucleic acid.

Primary pneumonia is characterized by rapid progression of fever, cough, labored breathing, and low oxygen levels that cause bluish skin.

[30] Influenza viruses also encode various accessory proteins, such as PB1-F2 and PA-X, that are expressed through alternative open reading frames[1][31] and which are important in host defense suppression, virulence, and pathogenicity.

[20] M1 and NEP proteins localize to the nucleus during the later stages of infection, bind to viral RNPs and mediate their export to the cytoplasm where they migrate to the cell membrane with the aid of recycled endosomes and are bundled into the segments of the genome.

[39] Transmission through contact with a person, bodily fluids, or intermediate objects (fomites) can also occur,[10][38] since influenza viruses can survive for hours on non-porous surfaces.

[17] Pneumonia caused by influenza viruses is characterized by high levels of viral replication in the lower respiratory tract, accompanied by a strong pro-inflammatory response called a cytokine storm.

[31] AIVs prefer sialic acids with an α-2,3 linkage, which are most common in birds in gastrointestinal epithelial cells[1][19][3] and in humans in the lower respiratory tract.

[1] There is, however, an "original antigenic sin", in which the first HA subtype a person is exposed to influences the antibody-based immune response to future infections and vaccines.

[2][49] Influenza can be prevented or reduced in severity by post-exposure prophylaxis with the antiviral drugs oseltamivir, which can be taken orally by those at least three months old, and zanamivir, which can be inhaled by those above seven years.

Nevertheless, strategies endorsed by experts for all phases of flu outbreaks include hand and respiratory hygiene, self-isolation by symptomatic individuals and the use of face masks by them and their caregivers, surface disinfection, rapid testing and diagnosis, and contact tracing.

[50] Reasonably effective ways to reduce the transmission of influenza include good personal health and hygiene habits such as: not touching the eyes, nose or mouth;[51] frequent hand washing (with soap and water, or with alcohol-based hand rubs);[52] covering coughs and sneezes with a tissue or sleeve; avoiding close contact with sick people; and staying home when sick.

[1] Diagnosis based on symptoms is fairly accurate in otherwise healthy people during seasonal epidemics and should be suspected in cases of pneumonia, acute respiratory distress syndrome (ARDS), sepsis, or if encephalitis, myocarditis, or breakdown of muscle tissue occur.

[1][46][62] Cultures can be used to characterize novel viruses, observe sensitivity to antiviral drugs, and monitor antigenic drift, but they are relatively slow and require specialized skills and equipment.

[62] Treatment in cases of mild or moderate illness is supportive and includes anti-fever medications such as acetaminophen and ibuprofen,[63] adequate fluid intake to avoid dehydration, and rest.

[64] Corticosteroids are not recommended except when treating septic shock or an underlying medical condition, such as chronic obstructive pulmonary disease or asthma exacerbation, since they are associated with increased mortality.

Later administration may still be beneficial for those who have underlying immune defects, those with more severe symptoms, or those who have a higher risk of developing complications if these individuals are still shedding the virus.

[46] The adamantanes amantadine and rimantadine are orally administered drugs that block the influenza virus' M2 ion channel,[1] preventing viral uncoating.

[10][1] Secondary S. aureus infection has been observed, primarily in children, to cause toxic shock syndrome after influenza, with hypotension, fever, and reddening and peeling of the skin.

[11] Neurological complications have been associated with influenza on rare occasions, including aseptic meningitis, encephalitis, disseminated encephalomyelitis, transverse myelitis, and Guillain–Barré syndrome.

[1][11] Influenza-associated encephalopathy can occur directly from central nervous system infection from the presence of the virus in blood and presents as sudden onset of fever with convulsions, followed by rapid progression to coma.

[1][11][2] In tropical and subtropical regions, seasonality is more complex and appears to be affected by various climatic factors such as minimum temperature, hours of sunshine, maximum rainfall, and high humidity.

Influenza C virus-associated hospitalization most commonly occurs in children under the age of three and is frequently accompanied by co-infection with another virus or a bacterium, which may increase the severity of disease.

To date, all known flu pandemics have been caused by influenza A viruses, and they follow the same pattern of spreading from an origin point to the rest of the world over the course of multiple waves in a year.

[77] In 1931, Richard Shope published three papers identifying a virus as the cause of swine influenza, a then newly recognized disease among pigs that was characterized during the second wave of the 1918 pandemic.

[23] In 1996, a highly pathogenic H5N1 subtype of influenza A was detected in geese in Guangdong, China[35] and a year later emerged in poultry in Hong Kong, gradually spreading worldwide from there.

[87] Other names that have been used for influenza include epidemic catarrh, la grippe from French, sweating sickness, and, especially when referring to the 1918 pandemic strain, Spanish fever.

[35] Instead, most transmission involves domestic poultry, mainly chickens, ducks, and geese but also a variety of other birds such as guinea fowl, partridge, pheasants, and quails.

Symptoms typically appear 1–3 days after infection and include fever, lethargy, anorexia, weight loss, labored breathing, coughing, sneezing, and nasal discharge.