The web-based software known as the Animal Health Emergency Reporting Diagnostic System, or USAHERDS, serves as a helpful digital tool for state governments to track and trace animal disease through populations of livestock. Now it's turned out to be a kind of infection vector of its own—in the hands of one of China's most prolific groups of hackers. On Tuesday, the cybersecurity incident-response firm Mandiant revealed a long-running hacking campaign that breached at least six US state governments over the past year. Mandiant says the campaign, which it believes to have been the work of the notorious Chinese cyberespionage group…