Which one do you want the least?
Painful, infectious, a classic.
When it comes to shear misery, it's hard to top Smallpox.
Violent diarrhea that can kill in hours.
This disease will melt you from the inside out. Get infected monday; by thursday you are projectile vomiting your own liquified organs.
Once in the bloodstream it has the highest mortality rate of any known disease (only one person in human history - Jeanna Giese - is believed to have survived an infection), and the symptoms are pretty nasty to boot: severe muscular spasms, agonising thirst arising from hydrophobia, frothing of the mouth, insanity, paralysis, and then mercifully, death.
People will be able to say you have Aids.