Most commonly Twilight, the first textually transmitted disease ever.
HOW TO GET A TTD.
If you you had Twilight, and you lent it to your friend, you gave your friend a TTD.
If you told your friend about Twilight and they bought it, you gave your friend a TTD.
If someone recommended Twilight to you, and you bought it (no matter how many months later), you have a TTD.
The only people who do not have TTDs are those who have not read the book yet, or those who saw it in the bookstore themselves and brought it without recommendation.
"Sorry, I lent it to Mike."
"DAMN IT! You give me a textually transmitted disease and you don't even give me a treatment?"