I wrote a short story about that.
In my story, there are about 39 Marcabs still alive, who were fortunate enough to be somewhere isolated from the plague. They've all been treated with the vaccine, of course. They're basically hiding out on the Marcab Homeworld under the protection of the Minbari, collecting all the DNA samples they can from the remains of the dead so that eventually they will be able to have a stable breeding population again. They're at least a century away from being at that point, though.
In return for their protection, they gave the Minbari all the jump-capable starships they had, save maybe one or two, and the Minbari stripped the Quantium 40 out of them and used it to build the White Star Fleet. 8)
That's my story, and I'm sticking to it.
Jon Acheson