Bird flu
Millions of chickens were killed in West Bengal, Assam, Orissa and Maharashtra recently because of the outbreak of bird flu or avian influenza caused by the H5N1 strain of flu virus. It is a viral respiratory disease that primarily infects poultry and other birds-migratory water birds, pet birds, and ostriches but humans can also be affected. The infection can be fatal as in severe cases people can suffer from acute respiratory disorders such as pneumonia, respiratory failure,etc. So the infected birds were killed to prevent the spread of H5N1 infection.