I'm wondering if random NPCs can become vampires after previously being human. There's a woman named Hert that owns Half-Moon Mill and just today while going to buy lumber for Lakeview Manor I was almost caught off-guard by her red glowing eyes. I never saw her with them before. She was also indoors during the entire day and I kept waiting around for her to appear outdoors so I could buy the wood I needed. I didn't want to picklock her door to enter her house to see if she was in there. After waiting the majority of the day I finally broke into her house and there she was just standing there with her bright red eyes. She always used to work the mill from 8am to 8pm but this time she was in her house all day, maybe to avoid the sunlight. I don't know.

So can random NPCs turn into vampires somehow?