This sickness was not in their bodies. It was in their ships. Their ships were organic. Their interstellar pilots were telepaths. And this is why humanity, with all our great achievements in science, remained bound to this tiny solar system.
Humans are, by nature, lousy telepaths.
We now know exactly how their ships work. Our physicists can explain to the most remote details how space is warped, how hyperspace is entered, and yet we cannot do it. There is something of conscienceness required. Humans can understand the process, but our brains can not perform the feat. We called these first visitors the Jaku Uchuusen , which means Fragile Spacecraft .