Alien- Endorphins

Lying back on the bed, he closed his eyes. “I feel normal. No, better than normal. I feel good, like I’ve just had a full night’s sleep and I’m ready to take on anything the day brings.”
It was true. He felt more than rested—he felt restored and rejuvenated, better than he had in years. He hadn’t felt this good since he’d been in his early twenties. Who needed a vacation?
...

“Interesting,” Brigette said. “The foreign body inside of you is causing a rise in your endorphin levels. Perhaps this is a self-defense mechanism. The better the host feels, the less likely that he or she will believe anything is wrong. They’ll continue to go about their business, and not even think about seeking treatment. It’s quite an elegant adaption.”
Foreign body? Hassan didn’t like the sound of that, but he felt too good to worry about it.

Prototype, Chapter 8