In 19th and Potomac, Jed asks Mrs. Landingham to drive back to the White House so he can tell her about his MS before he tells the public. That's when she gets hit.

Several episodes later, Sorkin's script added a lot of juicy dramatic tension into the background of her death. Here's a couple of pages from the scripts for Bartlett For America. There's a little tidbit that never made it to air.

Dolores overheard when Bartlett told Hoynes about his illness. So she never really had to go back to the White House that day. But Abby knew that Dolores knew. And Abby didn't tell Jed that she knew. Why not? Did Abby know why Dolores was coming back to the White House?

He added all that dramatic development retroactively, and in just one line "What does relapsing remitting mean?"