Bird flu was on Friday at Kolkata's doorstep with adjacent Howrah being declared affected besides Purulia as over half of West Bengal came in the grip of the deadly H5N1 virus. Blood samples of birds sent from Purulia's Saturi and Howrah's Sankrail areas to the High Security Animal Disease Laboratory in Bhopal and Pune tested positive, Animal Resources Development Secretary Dilip Chakraborty said.