Relativity, writes the author of this piece. "... transforms the thin wisp of hydrogen gas that permeates interstellar space into an intense radiation beam that would kill humans within seconds." She quotes William Edelstein of the Johns Hopkins University School of Medicine to the effect that Star Trek's Enterprise crewmembers would get over a thousand times the lethal dose of radiation.
COMMENT: Duh, that's what deflector shields are for :) .... seriously, I think that if you assume we can find ways to travel that fast, we can also solve these pesky side problems. It may take a couple of hundred years or even a couple of thousand, but, if I could bet with my future self, I would be willing to bet faster-than-light travel will eventually happen.