Here's a neat variation,
The disease is DNA altering retrovirus left by a dying race scattered throughout the galaxy. It was meant to recreate the race out of the genetic material of another race, but unfortunately, humanity wasn't a perfect match for the long dormant retrovirus. (The designers of the virus were methane breathers, for starters).
The disease spread as per Paul's original post, but the effects were varied. Some died outright, some in agony, some had minor annoying effects like odd skin pigmentation, odd bone or muscle growth. Some had their brains altered and in rare cases, turned into hyper intelligent beings, or more likely, regressed to Neanderthal levels of intelligence. Some descended into homicidal or suicidal madness, and some, some weren't recognizable as human any more.
No method of quarantine, no method of antisepsis worked. The retrovirus moved moved quickly via every method of infection known, within six weeks, some 65% of the global population was infected in some form or fashion and the varied effects lead to complete societal breakdown as people reacted out of abject fear of what new horror would come next.
Within six months, the world was quiet, except for the lucky few who were immune, and the infected souls who hadn't died either from the disease, or panicked people who killed the infected the first time they became symptomatic.
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