It’s not a disease, it’s an urban legend.
The Hanahaki disease was invented in the early 1990s by a group of scientists from Japan who wanted to show how contagious emotional mood could be. The story they devised claimed that the only way you could get this fictional illness was by receiving a letter containing your lover’s undying passion and love for you, but because of negligent safety regulations, someone had come before you and touched the stationary with their hand before sealing it inside an envelope. So when your letter arrived and you opened it, all sorts of hazardous waste were released into your bloodstream and caused deadly side effects as well as rare forms of cancer related to genetic disposition.