H5 viruses have long struck fear among scientists who study bird flu because of the economic damage they can wreak and their propensity to occasionally infect people. They are one of two subtypes — H7 viruses are the other — that can become highly pathogenic. That term refers to their deadliness when they infect birds. In the mid-aughts, H5N1 viruses decimated poultry flocks first in Southeast Asia and later into South Asia and North Africa.

H5 viruses have also been seen to infect people from time to time, often with devastating consequences. Of just over 860 previously recorded human cases of H5N1 infection — from 19 countries — just over half have died. For a number of years, it was feared the virus might cause a potentially devastating pandemic. But when the first pandemic in decades finally occurred in 2009, it was caused by a mild swine flu virus, H1N1.