H5N1 bird flu needs only 10 units of virus to infect a dairy cow through its udders, yet it refuses to spread through normal close contact.

Current containment strategies assume the bird flu outbreak in cattle is spreading through the air or shared water. Experimental tests show that while the virus is incredibly efficient at infecting mammary glands, it does not move between cows through milking equipment or respiratory droplets. This creates a massive mystery because the virus is clearly moving through farms in the real world. The tiny dose required for infection suggests that even trace amounts of contaminated material can trigger an outbreak. We are missing the primary way this virus moves, which makes it nearly impossible to contain right now.