Spoilers why couldn't Sean...
Just take a blood sample of Naya's infected blood, and killed her? Just put the sample of blood in a petre dish and freeze it? Why couldn't they do that?
Right now. You are no longer in control.
From what I remember, they did take a blood sample of Nyah's infected blood. They just didn't bother to kill her. Instead, they held open the possibility to let her loose in the city and cause a breakout of the disease, so that everyone would be in need of Bellerophon and hence their stock (or stock options) would rise in value.
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Joe Satriani - "Always With Me, Always With You"
http://youtu.be/VI57QHL6ge0
Yes, but they can use the blood sample and freeze it and have the disease in a petre dish.
Right now. You are no longer in control.