Influenza is divided into 3 types: A, B and C. Type A influenza includes most human and all avian influenza viruses, while influenza B almost exclusively infects humans.
Wild aquatic birds are the natural hosts for a large variety of influenza A viruses. Once in a while viruses are transmitted to other species and may cause disatrous outbreaks in domestic poultry or lead to human influenza pandemics.
The type A viruses are the most virulent human pathogens among the three influenza types and cause the most severe disease.
Influenza viruses are divided into subtypes based on the two proteins, haemaglutinin (H) and neuraminidase (N) that they have on their surfaces.
The types of Influenza A that have been confirmed in humans:

Others include H9N2, H7N2, H7N3, H10N7.

Influenza virus changes often, so having flu caused by one strain does not give human full immunity to other strains.
Widespread outbreaks of the flu usually follow significant changes (antigenic shifts) in the virus and occur about every 10 years. People who get the flu tend to become much sicker when a shift in the flu virus occurs.
Minor changes in the virus (called antigenic drifts) occur nearly every year.

Symptoms top
Symptoms can appear in sudden, about one to two days after infection. First symptoms are usually chills or a chilly sensation. Fever may occur with body temperatures as high as 39 °C (approximately 103 °F).
Patients sometimes have aches and pains throughout their bodies, which are worst in their backs and legs.
People who have the flu are most likely to pass it to someone else from 1 day before to 5 days after symptoms develop. Children may be infectious for up to 6 days before symptoms develop.
Symptoms in later stage may include:
‧ Body aches, especially joints and throat
‧ Coughing, sneezing and nasal congestion
‧ Extreme coldness and fever
‧ Fatigue and headache
‧ Irritated watering eyes
‧ Nausea and vomiting
‧ Reddened eyes, skin (especially face), mouth, throat and nose
It can be difficult to distinguish between the common cold and influenza in the early stages of these infections, but usually the symptoms of the flu are more severe than their common-cold equivalents.

Cause top
Channels of human-to-human transmission
‧ Direct contact, e.g. shaking hands 
‧ Small droplets from sneezes or coughs 
‧ Contact with objects like handkerchiefs that have been in contact with fluids from an infected person's nose or throat.

Prevention top
Vaccination is strongly recommended for high-risk groups, e.g. children and elderly.
Develop good personal health and hygiene habits.
Cover mouth while sneezing and wash hands regularly.
Surface sanitizing is recommended in areas where influenza may be present on surfaces. Alcohol is an effective sanitizer against influenza viruses. Quaternary ammonium compounds can be used with alcohol to increase the duration of the sanitizing action.