In March researchers at the U.S. Department of Agriculture received a shipment of what they thought was a relatively harmless strain of animal flu from the U.S. Centers for Disease Control and Prevention. It was, in fact, highly pathogenic H5N1 flu virus. Months later the researchers only began to suspect something was amiss when their experiments yielded results that they didn’t expect and couldn’t explain. The CDC, it turns out, had sent the deadly samples inadvertently.