Knowing my love of overanalyzing small aspects of a series' universe, I sometimes imagine what an in-universe medical research paper on space disease would look like, and having a sort of grounded in reality look on how it affects the body.
Considering the 1978 Harlock anime's flashback episodes don't refer to space disease by name but rather described by Tochiro as his "anemia acting up", it's safe to assume that space disease is a blood disorder, if not a cardiac disorder or both, but very likely affecting blood cell count or even affecting the hemoglobin on top of that. While it's incurable in the SSX timeline, and perhaps the disease is unknown in the 1978 timeline and assumed to be a different disorder (or very well COULD be an irl disorder causing anemia in that timeline), medications DO exist that slow down its progression while treating flare ups, much like treatments of some irl diseases and disorders, like beta blockers that treat heart problems or medications designed to slow the progression of certain cancers. (though the medication we do see Tochiro use in SSX seems to be more for treating flare-ups, same goes with the med Kei Yuki's dad is seen taking for his heart issues).
Basically, the only option is to treat the symptoms and hope the disease takes one's life later rather than sooner, and perhaps slow the progression as long as one can. And for all we know, that could also be what Kei Yuki's dad was suffering from in his only appearance in SSX, further emphasizing its affects on the heart as well as the blood.
It's clear the disease has been known about for quite some time. Perhaps it was discovered in the 29th century if not a couple centuries after humanity started settling in other planets. Surely that wormhole in SSX ain't the ONLY place people have contracted space disease from.