SPOILERS AHEAD for Supergirl Season 3 Episode 16: "Of Two Minds."
Blight comes to National City, first hitting pigeons before it takes over City Hall. The blight is 100% fatal, and does not seem to be airborne, but once you show symptoms, it is too late to treat it. While examining the ill at City Hall, Supergirl notices a scratch on the back of each victim's hand, and decides that is how the disease is transmitted, though later it appears that the scratches don't appear until other symptoms become present.
Imra takes it upon herself to seal City Hall in a blue force field, for fear that Pestilence might escape. And so begins the battle between Imra and Supergirl. Imra is there for one goal: to kill Pestilence and thereby stop the blight from wiping out future generations. Supergirl is deeply opposed to this, as she believes they can reach out to the woman who is being "held hostage" by Pestilence and get her to fight back.
Though the bickering between the two superheroes becomes a little tiresome, it does lead to an interesting question, which is not that different from the age-old question: If you had the chance to kill Hitler before he set his genocide into motion, would you? From the way Imra describes Pestilence, that is basically what she would be doing: killing Hitler. The question is not posed in such a way to Supergirl, but it seems like it would have been a good way to put it into context for her.
Back at the DEO, Winn falls ill with the blight. The cure is coded into our time travelers' DNA, so Supergirl instructs them to get started on creating the cure while she goes looking for the most likely suspect: an assistant named Adelaide who passed out cookies to all the infected. By the time Supergirl finds her, Imra has already gotten there, and Adelaide is dead — though not by Imra's hand. Adelaide was human, and succumbed to blight before Imra arrived.
Mon-El seems to side with Supergirl, which has got to piss off Imra. He wants to at least try it Supergirl's way, and as we later find out, at least part of Imra's quest is due to the fact that her sister dies from the blight, and she wants to save her. Interestingly, when Imra eventually admits this to Supergirl, she describes her sister as someone who was quiet and kept to herself. A strange choice of words, since those are the terms most often used to describe a serial killer.
Anyways, it turns out the "cure" isn't working because it is created for a different version of the blight, and in order to change it, Brainy needs DNA from Pestilence directly, so Supergirl has to act fast. Adding to the urgency is that Alex has now taken ill. Citywide cameras reveal nothing, so Supergirl turns to social media and finds one common denominator between all the victims: Dr. Grace Parker. She had been on the scene when the City Hall outbreak first began, and interacted directly with both Winn and Alex. I thought she was just flirting with Alex, but it turns out she is an evil alien-god.
Supergirl finds Dr. Parker at a health insurance company, about to infect the bigwigs for denying health coverage to their customers. (Can you blame her?) Supergirl tries to use the doctor's code, "do no harm," to get through to Dr. Parker, while Mon-El convinces Imra to hold back and just let Supergirl try. Supergirl's method seems to be working, and Dr. Parker de-Pestilences herself — but only for a moment. Pestilence comes out in full force, and an all-out alien fight breaks out.
Imra manages to lock Pestilence in a force field and shoots her with some kind of bullet-thing. This seems to be causing damage to Pestilence, and she weakens. Unfortunately, Purity finds her, and being near her strengthens Pestilence. The two fly out of the building together. On the upside, the bullet-dart-thingie is left behind, and with it, enough DNA for Brainy to upgrade the cure.
Meanwhile, over at L Corp, Lena is desperately trying to get to the root of Sam's Reign problem. The only way Lena can figure out what Reign's trigger is is to bring her out. And the best way to do that is with pain. They go through several courses of electrocution, and each time, Sam goes into what she calls the Dark Valley — basically like the Upside Down — where she meets Reign, and Reign tries to convince her to come to the dark side. She threatens Ruby, which really upsets Sam.
At the DEO, Winn picks up Purity and Pestilence's signatures... heading straight to L Corp. Supergirl, J'onn, Imra, and Mon-El race over there, just in time to see Pestilence and Purity crash through the roof and spring Reign from her prison.