Girl sees ghost/apparition of boy living through Black Death

I read this book as a kid and have been looking for it since. From what I remember, a teenage girl in modern times sees apparitions of an older boy who lived in the dark ages, during the Black Death.

Can't remember if she travels through time or just sees flashes of his life in her dreams.

I remember his story is quite tragic. I want to say he was called Seth but probably not. The cover may have had a girl holding a candlestick going up stone stairs. There's a scene with kids dancing in a circle holding hands and singing the "ring a ring o roses, pockets full of posies" song.

Sorry for the terribly vague description! I've tried searching for everything I can think of...

The book was small and thin (think like one of the old Goosebumps book, thinner than the 1st Harry Potter). It would have been published in the 80s, 90s or very early 2000s. Any help is much appreciated!