Darrin gaped at Nicholas with unmasked shock and worry. And fear. He had the virus. And he was right there in the house. He could lunge for someone any second, and the fact that Mattie wanted in the guy's pants or whatever only made it worse. The hairs on the back of his neck prickled. 

"So the guy's infected and you're just letting him lounge on the couch," he said grouchily. "Great idea. I can see why you've survived this long." He addressed Nicholas coldly, now: "Oh, and by the way, stay away from my sister. I've already lost my only brother, and you're not going to infect her with your nasty virus." 

Mattie came back after having rocked Cole to sleep. "Maybe you guys should give Nicholas a little space," she suggested, oblivious to Darrin's warning.