(CNN)Here's a look at avian flu.
Avian influenza, also called avian flu or bird flu, is an illness that usually affects only birds.
There are many different strains of avian flu: 16 H subtypes and 9 N subtypes. Only those labeled H5, H7 and H10 have caused deaths in humans.
The most commonly seen and most deadly form of the virus is called "Influenza A (H5N1)," or the "H5N1 virus."
Most cases of human bird flu infections are due to contact with infected poultry or surfaces that are contaminated with infected bird excretions: saliva, nasal secretions or feces.
Symptoms of avian flu include fever, cough, sore throat and sometimes severe respiratory diseases and pneumonia.
The CDC recommends oral oseltamivir (brand name: Tamiflu), inhaled zanamivir (brand name: Relenza) and intravenous permavir (brand name: Rapivab) for the treatment of human illness associated with avian flu.