"The cops get this report to stop a red Jackrabbit with tag thus-and-so," the doc goes on, "and the car’s the virus. Okay?" I nod. No-brainer so far. "In an antigenic shift, it’s like the red Jackrabbit keeps changing its tag and its color," she continues. "The cops keep getting updated reports—now it’s a black Jackrabbit with out-of-sprawl tags, that kind of thing—but they’re always one step behind. That’s antigenic shift, and that’s what this virus is doing." She frowns. "Sort of."