Kara and the D.E.O. were chasing Pestilence in “Of Two Minds.”
The bad news was that their cure was for a later stage of the disease and it could not be tweaked without a sample of Pestilence’s DNA.
Winn and Alex got sick, which raised the stakes for everybody at the D.E.O. The good news was that the members of the Legion had the cure to the “blight” written into their DNA, and that could theoretically be used to save the people who were getting sick.
Birds dropping dead out of the sky at the end of the previous episode were already a big clue that she would be trouble when she turned up, and when people started getting very sick in “Of Two Minds,” it quickly became clear that Pestilence needed to be stopped.