How it spreads
Avian influenza A (bird flu) viruses may be transmitted from infected birds to other animals, and potentially to humans, in two main ways:
Directly from infected birds or from avian influenza A virus-contaminated environments.
Through an intermediate host, such as another animal.
Direct infection can occur from exposure to saliva, mucous, or feces from infected birds. Bird flu infections among people are rare; however, human infections can happen when enough virus gets into a person's eyes, nose, or mouth, or is inhaled. People with close or prolonged unprotected contact (not wearing respiratory and eye protection) with infected birds or places that sick birds or their mucous, saliva, or feces have contaminated, might be at greater risk of bird flu virus infection. Link