[It's a practical question—and seemingly an earnest one, coming from Ninety-Nine. If Reiju is next to suffer a bite that can't be cordoned off, at least one person should know how to put her down. Other intentions aside, this is perhaps the biggest reason she answers easily:]

My exoskeleton is as durable as metal. Pierce that, and I'll be susceptible like anyone else. Then it'll just be a matter of killing me before I can recover.