So, in my story, a main character gets sick with a virus. (Not a real one). I alternate POV's, and she is one of my favorite to write. I'm pretty sure her mindset would change slightly. But how? Please help!
Put yourself at her place !
You can make her first observe some symptoms, without paying a real attention, it has to be small details, you don't even have to make it like she is noticing, like she sees something in her eye but she thinks it's because she did'nt sleep enough, or she feels weak that afternoon, or her cat acts strangely in her presence etc.
But slowly bigger symptoms will appear, ans she will begin to suspect something !
Also, are you writing a realistic story or a SF story ?
If I correctly read your question, its more about the voice and pov.
You can research symptoms and bring it out in forgetfulness, repetition, misidentification etc
A knock on the door, it was a salesman. What does he want to sell me? It was the third one this week.
"Mom, are you okay?", asked the salesman