The precipitating event is in “He Shall, from Time to Time.” The president comes down with the flu and Leo McGarry notices that Abbey Bartlett has an unexpectedly extreme level of concern over it (cutting a trip short and talking about post-poning another even though the fever is going down). He confronts her about it, and she admits that the president has multiple sclerosis:
ABBEY
Leo, I came by because I wanted to ask do you think there's a huge downside
to postponing for a few days?
LEO
Did the fever go back up?
ABBEY
No, it's going down.
LEO
Abbey? What's going on?
ABBEY
Nothing. Uh, like I say--
LEO
What should I know that I don't know?
ABBEY
I just thought we should be on the safe side.
LEO
Why'd you cancel your trip?
ABBEY
Because he has the--
LEO
Because that President has a temperature. The President's not in nursery
school.
ABBEY
Look, if it's a problem to postpone--
LEO
It's not a problem to postpone. Of course, I wouldn't do anything to jeopardize
the President's health. What I'm saying...
ABBEY
Leo, please.
LEO
[sits] Hey, hey, hey. This is me. This has happened before. I see you trying
to cover the panic. I see you prescribing medication. I think you're giving
him shots. What does he have he can't tell people?
ABBEY
He has the flu.
LEO
Ugh! You would not have come back for the flu, Abbey.
ABBEY
He fainted. He was running a fever.
LEO
Abbey?
A pause. Almost in tears, Abbey decides to tell him the truth.
ABBEY
He has multiple sclerosis, Leo.
LEO
[shocked] Oh, Abbey.
ABBEY
A fever could be life threatening.
Abbey tries hard to fight back tears, but couldn't. A tear falls from her eye.
Now Leo knows about the MS, and the cat is out of the bag. Previously, only Jed and Abbey Bartlett knew about it and so kept it a secret, but once the inner circle starts expanding, it becomes harder to keep—and the risk to those who know, should it ever be publicized, becomes greater. After all, Leo figured out something was up, and the President is probably the most scrutinized figure in the world—if Leo did, it seems like only a matter of time before someone else does. Sure enough, it turns out that C.J. had seen something, though unlike Leo she did not confront the Bartletts about it.
A major theme with this portion of The West Wing is the President’s guilt about the entire thing. When only Abbey and Jed knew about it, they were responsible only for the consequences to themselves (or at least so they told themselves). They could rationalize their behavior, feel they were only putting themselves at risk and taking responsibility for their actions and lives. But now, because he knows, Leo could be in major trouble if he kept this secret. Jed does not want to do that to a friend, to put a friend in that position. Coming clean is the only way to save Leo from having to risk it. It takes some time to admit it, but Jed’s conclusion in “H. Con-172” (S03E10) becomes more and more inescapable as the issue is discussed: “I was wrong.”