[8] Further variations exist within the subtypes and can lead to very significant differences in the virus's ability to infect and cause disease, as well as to the severity of symptoms.

[9][10] Symptoms of human seasonal flu usually include fever, cough, sore throat, muscle aches, conjunctivitis and, in severe cases, breathing problems and pneumonia that may be fatal.

[12][13] Bird-adapted strains of the virus can be asymptomatic in some aquatic birds but lethal if they spread to other species, such as chickens.

Some examples include:[28] The influenza A virus has a negative-sense, single-stranded, segmented RNA genome, enclosed in a lipid envelope.

[30] The nucleoprotein (NP) coats the viral RNA to form a ribonucleoprotein that assumes a helical (spiral) configuration.

Three large proteins (PB1, PB2, and PA), which are responsible for RNA transcription and replication, are bound to each segment of viral RNP.

Two viral proteins; hemagglutinin (HA) and neuraminidase (NA), are inserted into the envelope and are exposed as spikes on the surface of the virion.

The M2 protein forms an ion channel in the envelope and is responsible for uncoating the virion once it has bound to a host cell.

[30][31][32] The table below presents a concise summary of the influenza genome and the principal functions of the proteins which are encoded.

At a later stage of infection, newly manufactured viral RNA segments assemble with the NP protein and polymerase (PB1, PB2 and PA) to form the core of a progeny virion NA also facilitates the movement of infective virus particles through mucus, enabling them to reach host epithelial cells.

[56] Annually, about 3 to 5 million cases of severe illness and 290,000 to 650,000 deaths from seasonal flu occur worldwide.

[60] As of June 2024, there is concern about two subtypes of avian influenza which are circulating in wild bird populations worldwide, H5N1 and H7N9.

Both of these have potential to devastate poultry stocks, and both have jumped to humans with relatively high case fatality rates.

[64] Annual vaccination is the primary and most effective way to prevent influenza and influenza-associated complications, especially for high-risk groups.

[68][69][70] The main treatment for mild influenza is supportive; rest, fluids, and over-the-counter medicines to alleviate symptoms while the body's own immune system works to recover from infection.

[75] The onset of symptoms is sudden, and initial symptoms are predominately non-specific: a sudden fever; muscle aches; cough; fatigue; sore throat; headache; difficulty sleeping; loss of appetite; diarrhoea or abdominal pain; nausea and vomiting.

[77] Symptoms of avian influenza vary according to both the strain of virus underlying the infection, and on the species of bird affected.

Symptoms of influenza in birds may include swollen head, watery eyes, unresponsiveness, lack of coordination, respiratory distress such as sneezing or gurgling.

Chickens infected with LPAI display mild symptoms or are asymptomatic, whereas HPAI causes serious breathing difficulties, significant drop in egg production, and sudden death.

[80] Since 2006, the World Organization for Animal Health requires all detections of LPAI H5 and H7 subtypes to be reported because of their potential to mutate into highly pathogenic strains.

Swine flu subtypes are principally H1N1, H1N2, and H3N2;[82] it is spread either through close contact between animals or by the movement of contaminated equipment between farms.

[86] Clinical signs of equine influenza include fever, nasal discharge, have a dry, hacking cough, depression, loss of appetite and weakness.

[87] Most animals infected with canine influenza A will show symptoms such as coughing, runny nose, fever, lethargy, eye discharge, and a reduced appetite lasting anywhere from 2–3 weeks.

[88] The H3N8 strain has evolved from an equine influenza avian virus which has adapted to sustained transmission among dogs.

[88] It is likely that the virus persists in both animal shelters and kennels, as well as in farms where dogs are raised for meat production.