Avian influenza (bird flu) is a type of influenza A virus that is lethal to poultry and is potentially fatal in humans. Birds are natural hosts for all types of avian influenza viruses, and usually, carry it without showing any symptoms of the disease but spread it to domesticated birds. There are many subtypes of avian influenza viruses, but only some strains of five subtypes have been known to infect humans: H5N1, H7N3, H7N7, H7N9, and H9N2. The virus transmits between birds through secretions from the nostrils, mouth, and eyes of infected birds and to people through direct contact with infected poultry, such as during slaughter or plucking. Monitoring and controlling avian influenza at its poultry source are essential to decrease the virus load in susceptible avian species and the environment. Diagnosis is by ELISA to detect disease-specific antibodies, viral isolation, or detection of the viral genome.