The bubonic plague was a painful disease, with black buboes, or swellings, in the groin and armpits, which lasted up to a week. There was some chance of surviving if the buboes burst.
As of 2015, full retirement age is 67 if you were born after 1960. The amount you receive doesn't change because disability benefits were calculated based on what you would receive at retirement age. However, you may see an increase if Social Security provides a cost of living increase.
You can apply for Social Security disability benefits when you become disabled, even on the day you become disabled. What is important, and where the 12 months comes from, is that your disability must be expected to last at least 12 months (or result in death).