I can spread like a disease,
Only I can't kill,
Unless it's in a phrase.
I can never displease,
Unless you've got skill.
But, I can still infect for many days
What am I?
Hint 1: Might give it all away:
If you add a letter to me, then I suddenly become more ghastly.
Hint 2: In case you need another:
No one can control me and they love it. Even psychopaths