How they found a cure.

My version.

A Drakh agent, fatally wounded, accidentally contracts the plague with the Excalibur's crew in pursuit. He gets to his ship, takes off, BUT he doesn't enter hyperspace. Just heads out of the system at sublight.

He dies from his wounds and an autopsy says the plague became inert for some reason.

They found the inert virus and discovered it had a failsafe in the event it infected the Drakh, and it has something to do with solar energy or some external energy source.

Which of course the Excalibur finds and destroys in an epic fight. Only like on Yamato the Drakh tries to destroy the Earth for the final time out of spite which wipes out the race.