More snow geese suspected to be infected with highly contagious bird flu have been found dead in the Lehigh Valley, where Pennsylvania wildlife officials announced the likely presence of the virus among the migratory waterfowl at the beginning of January.

Avian flu is a common illness among birds, but it can spread to humans and other mammals. The virus is shed through excretions, such as saliva and feces. Exposure to sick or dead animals is the primary way humans contract bird flu, according to the CDC.