Hillary Clinton collapsed at the 9/11 memorial service on Sunday, delivering another jolt to the already chaotic 2016 Presidential campaign. The incident means Clinton’s health, long a fixation of conspiracy theorists, is now a bona fide campaign issue and raises the question of what happens if she cannot continue.

To be clear, the Clinton campaign has not said anything remotely indicating she will quit the race. Nor is there evidence the incident on Sunday was related to anything other than a temporary bout of pneumonia.

The Clinton campaign says more information about the candidate’s medical records will be coming soon but “there’s no other undisclosed condition; the pneumonia is the extent of it,” Clinton spokesman Brian Fallon said on MSNBC’s “Andrea Mitchell Reports.”

But given what’s at stake, here are the rules for how the Democrats would pick another candidate.

As historian John Buescher explains in a helpful blog post, it’s the job of the Democratic Party leadership to replace Clinton if she drops out prior to Election Day. Per the party’s by-laws, this would involve the Chairman calling a meeting and a majority of Committee members picking someone else — Tim Kaine or Joe Biden or, well, someone — to be the nominee. Already the guessing games have begun:

It gets a bit more messy if a nominee dies or steps down after Election Day but prior to the time the electors (the people voters choose to convey their choice for President under the electoral college system) meet to vote for President. In that unlikely event, then the process is the same — the Democratic party bigwigs pick who the new nominee would be; this outcome might be more unpredictable, however, since it would involve the choice of someone who is not on the ballot.

Finally, if the Democratic nominee dies between the time of the electoral college vote and the Presidential inauguration, then the Constitution’s 20th Amendment kicks in. It states the Vice President replaces a President who dies.

As Buescher notes, these processes evolved as the country came to grapple with scenarios that the Founding Fathers didn’t foresee. And while it’s very rare for candidates to leave for health reasons, it’s not unprecedented. During the 1972 campaign, Democratic candidate George McGovern’s running mate, Thomas Eagleton, dropped out after revelations he had been hospitalized for depression and stress. The DNC then met in order to replace with Peace Corps Director, Sargent Shriver but McGovern lost to Richard Nixon in a landslide.