Young Adult book that was probably published in the 80s. It was about two siblings. They live in a small town by the ocean and I believe that after they find a dead sea gull a mysterious illness starts to spread through the town.

[Maggie]

MORE DETAILS: (POSSIBLE SPOILERS)
Once infected the kids can't come out at night and no one can figure out what is going on with the kids. I believe the siblings were brother and sister and there dad was a doctor. I remember that the brother wanted a cat and the dad said no because he believed the carry diseases. Also, I THINK the brother's best friend was the first to be infected with the mysterious illness. I want to say something weird happened with their eyes and that was why they couldn't come out during the day or have any lights on in their rooms.

[Ayshe]

The Night Walkers maybe?

(Probably the same book as the one searched for here: https://www.goodreads.com/topic/show/... )