Spoiler alert!!!
Don't read if you don't like spoilers, just skip this comment
The reason why Joel/Lin You is getting sicker because of snake demons latched onto him since he was little and it takes Lin You's life energy bit by bit and makes nightmare go up until it reaches 100% and why does the snake demons has similar appearance with Mara/Yikui? Because he's the one who latch the snake demons on Lin You.
That's the only spoiler I can give you because chapter 80 hasn't been released yet and it will be released on July 12th at
Loose Teens, doing our thing
We got the whole world in our hands
And baby, Loose Teens making the means
To make the world our number-one fan
Loose Teens!
Loose Teens!