At the DEO, Maggie was being tested. Through glass walls, Alex could see Maggie lying on a table, numbers of scientists and DEO's finest doctors attending to her. Her face was shiny with sweat, her dark blue FBI jacket was already removed, and her collared shirt was now soaked with sweat too. She looked so small underneath all the equipment. There was no sarcasm, no fiery spirit, and all that confidence that usually emanated from her just seemed…gone. Alex ran a hand through her hair and forced herself to turn away. She had been ordered to stay outside the medical bay for fear that whatever was wrong with Maggie was contagious. Alex couldn't even think of what would be wrong, but the knot in her gut hadn't loosened yet. Something was wrong.

J'onn watched as Alex forced herself to leave. He remained on the outside of the glass box, his arms across his chest. He was stumped as to why Maggie was in this state. What would cause a human to be so sick, so weak that a doctor would not be able to identify the ailment? Finally, J'onn sighed and left to find Alex. He found her in the upstairs lab, sitting hunched over on a stool. She didn't move when he approached.

"Can I come in?" J'onn asked, his hands behind his back, his head bent.

"Of course, Sir," Alex said and pushed her stool away from the table.

"What are you doing in here, Alex?"

"I'm not sure," she shrugged. "I couldn't be in the room with Maggie, so I guess I just came to the room where I knew everything." J'onn tilted his head. "You know, this lab. I know where every chemical is. I know where the equipment goes and how to use it. It's all known. But this," she pointed vaguely in the direction of the medical bay, "I have no freaking clue. About any of it."

"You wanted to be somewhere where you feel like you have just a little bit of control," J'onn narrowed his eyes, nodding.

"I just want to know what's happening," Alex sighed. "I don't like this, J'onn. I've never felt this anxious before."

"We'll figure it out," J'onn said as he put a hand on Alex's shoulder.

"Agent Danvers."

Alex looked up, her face lifting. "Doctor Monroe. What's going on?"

"It seems to us, after checking every possible human disease, that Ms. Sawyer does not in fact have a human disease."

"What do you mean?" Alex's eye narrowed, her arms folding over her chest.

"Ms. Sawyer has contracted a severe alien disease."

Alex was silent.

"An alien disease?" J'onn asked.

"Yes," Doctor Monroe said. "We haven't seen anything like it. It's a virus that can be contracted by coming in contact with a Patogenian. Where would Ms. Sawyer have come in contact with one of those?"

Alex sighed, closing her eyes. Why didn't she see it before? "She—Maggie—said she had gotten a new case today. It was an alien homicide. I'd have to see the reports, but the victim may have been a Patogenian."

"If that's true, then no one in proximity to the body is safe," Doctor Monroe said.

"What exactly does this disease do?" J'onn asked.

"It's caused by an invisible gas the body lets off as it starts decomposing. If Ms. Sawyer inhaled these fumes, then the toxins are already spreading through her bloodstream."

"You didn't answer the question," Alex clenched a fist and almost growled. "What does it do?"

"The toxins start as a paralytic," Doctor Monroe said slowly. "Then, as time goes on, the paralytic reacts with the plasma in the blood and becomes a parasite. It will break down any internal tissue while a foggy blue poison spreads throughout the body. The tissue decomposition will complete first or the poison will reach Ms. Sawyer's heart, but either way, without an antidote, the outcome is not good."

Alex opened her mouth to say something, anything really, but she ended up standing there with her jaw half open.

"Alex," Kara burst through the lab doors, out of breath, her eyes wide and frantic. "I'm so sorry! I couldn't get away from work, but J'onn called me and told me what happened. How can I help?"

"What do you know about Patogenians?" J'onn asked.

"The Blue Blood aliens?"

"Blue Blood aliens?"

"Well, that's what Kryptonians called them," Kara explained. "Their signature mark is their blue blood, which doesn't show up until after death, and those who are infected after coming in contact with a body are left with blue toxin-filled blood—" she stopped suddenly, her mouth still open. Her eyes widened and she shook her head. "Please tell me Maggie didn't get near a Patogenian."

"We're not sure because we haven't seen the police reports, but yes, that's what we're thinking," J'onn said.

"Oh, Alex," Kara sighed, walking over and putting her arms around her sister. "It's okay. I'm gonna do everything I can to help."

"How long do we have?" Alex whispered, the first words she'd said in a while.

Kara looked at J'onn and Doctor Monroe and both men nodded. "The toxins work fast. We probably only have about eighteen hours now."

Alex's breathing hitched, but only Kara could feel it. "Let's go. What do we have to do?" she asked as she pulled away from Kara.

"We need the body," Kara said. "The same toxin, uncontaminated within the body, if treated correctly, can become an antidote."

"I'll drive," Alex said.

"No, I'll fly us there," Kara said, already ripping open her blouse. "It'll be faster."

"Special Agent Danvers with the FBI," Alex shouted as she entered the police station. "We need to see a recent homicide victim."

A man in NYPD uniform came up to Alex and Kara who was dressed as Supergirl. "On whose authority?"

"On the authority that you may have a possibly life-threatening and contagious alien in your morgue," Supergirl said, her hands on her hips. "We have to make sure everyone here is safe from the pathogen."

The officer's eyes widened and he nodded. "Follow me."

He led them back down the hallway and down to a lower level, which instantly decreased in temperature. Down the hallway ten steps and then a door on the left and they were in the coroner's office. Immediately in the room, on a silver table, was the Patogenian. He was pale, his skin starting to turn back to its original blue hue, but to most humans it would just look like the stages of postmortem.

"That's him," Kara nudged Alex's shoulder. "That's the Patogenian."

"Alright," Alex said to the officer. "I need a list of everyone who has come in here while this body was here. Find them and bring them to me. They need to be examined to make sure they haven't contracted the pathogen."

"Everyone?" he asked. "Do you know how many people—"

"Their lives are at stake," Alex tilted her head and took a step closer. "Do you want to be responsible for their lives?"

"N-no ma'am," he shook his head. "I'll find the sign-in sheet."

"Thank you," Kara nodded. "Let's talk to the coroner," she pointed to the door to the left of the autopsy table.

Kara walked over and knocked gently on the metal door and waited for an answer. An older gentleman opened the door, a half-eaten sandwich in his hand.

"Hello," he said. "Uh, how can I help you two?"

"Special Agent Danvers with the FBI," Alex said, flashing her ID, "here to speak with the head coroner."

"Well, that would be me. Doctor Bishop," he chuckled, looking down at his sandwich and then at his hand. "Uh, I'm sorry you've caught me at my lunch break."

"It's fine," Kara said. "We needed to know about your findings with the victim on the table over there."

"Oh," he said, shrugging. "Nothing out of the ordinary except for the cause of death."

"Nothing out of the ordinary?" Kara asked.

"No," he said slowly.

"Have you seen how blue his skin is?" Alex asked, a little more aggressively than she had intended.

"His skin? There's nothing wrong with—oh my," he said, dropping his sandwich on the ledge next to him. "What's happened?" he asked, pushing past Alex and Kara.

"It seems you've ended up with an alien homicide," Kara said. "We need to take custody of him because this species releases a toxic gas postmortem."

"An alien?" he asked, not particularly to either of them. "How did I not see this?" He spun around, pointing. "This is my job, you know. I'm supposed to know the human anatomy so that if anything's amiss, I know. That's my job…" he seemed aghast.

"Please don't beat yourself up, Dr. Bishop. This type of alien has identical human anatomy up until time of death," Kara said. "However, we don't have time to discuss this with you. We need this body because we're afraid it has already affected people here."

"What should I do?"

"You'll be met by one of our agents who will bring you back to headquarters to make sure you're not infected, and if you are, you'll be treated with an antidote," Alex said.

"An antidote?" Dr. Bishop asked. "How serious is this exactly?"

"It's nothing you have to worry about," Alex shook her head as she placed her hands on her hips. "Just cooperate and you'll be back to doing your job in no time."

"Of course," he nodded. "Whatever you need."

Alex moved away and lifted her cell phone to her ear. "I'll call for the quarantine team that's waiting outside the precinct. You take care of the body."

"Uh, Alex," Kara said under her breath.

"What?"

"I can't actually touch the body."

"What do you mean?"

"I'm an alien yes, but Patogenians are deadly to Kryptonians too. A huge amount of our population was wiped out hundreds of years ago because of a war with them. We didn't know anything about them. Innocent people, people just doing their job, were killed," Kara said.

"And you came anyway?" Alex whispered loudly. "What the hell, Kara!?"

"I can help," Kara said. "If I freeze his body, that may trap some of the toxins leaking through his skin. Or I can hold my breath."

"All the way to the DEO?"

"Sure," Kara shrugged. She tried to pull off a confident shrug, but it came off more as uncertainty.

"I can't have you contracting this thing," Alex whispered, stepping closer.

"And I don't want you getting it either, but we both have jobs to do," Kara said. "I'll be as careful as I can. I promise."

Alex sighed and looked down at her boots. It was a moment before she looked Kara in the eyes. "Alright, but you're getting tested as soon as you get the body back."

"You too," Kara said as she stepped closer to the Patogenian. "And after all this, we have an alien killer to catch."

"One thing at a time."