DES MOINES, Iowa (AP) — It’s been five months since the H5N2 bird flu virus was discovered in the U.S., and producers have lost more than 21 million birds in the Midwest alone. Yet, researchers with federal agencies acknowledge they still know little about the virus’ origin and how it spreads, especially with heightened biosecurity measures at commercial poultry farms and the apparent lack of widespread deaths in largely unprotected backyard flocks.

A leader with the federal Centers for Disease Control and Prevention says not much is known about the virus because it only surfaced in the U.S. in early December. Unanswered questions include how exactly the virus finds its way into sheltered commercial chicken and turkey flocks and if wild birds spread the virus why more backyard flocks haven’t died.