Latest word (via ABC News) is that Senator Johnson suffered a stroke -- that it is non life-threatening, but that the "extent of impairment is unknown." Assuming that report is accurate -- and other reports only report "stroke-like symptoms," so it's hard to know for sure -- what does it mean?
Stroke treatment has advanced enormously in the last few years. The most advanced (and somewhat controversial) therapy, known as Tissue Plasminogen Activator or TPA, quickly dissolves the clot obstructing blood flow to the brain. But it must be administered within a short time span of the first observed stroke symptoms. (Two hours, as I recall, though I'm not certain about that.)
The good news is that it sounds like paramedics quickly took Johnson to George Washington University, a teaching hospital with a dedicated "stroke team" that almost certainly has such technology available.
With or without TPA, the range of recovery from stroke varies enormously, depending in part on the severity of the stroke itself. In the worst cases, stroke victims who survive suffer paralysis, loss of speech, and so on. But in the best cases, they can experience a full, nearly 100 percent recovery, sometimes with only very minor rehabilitation.
Here's hoping (on a purely human, non-political, level) that Senator Johnson is one of the fortunate ones who fall into the latter category.