In the Rule entitled “Viral Proposals”, change the text

Viral Proposals can only be created by other Proposals or by rules that explicitly state that they create Viral Proposals.

to

Viral Proposals can only be created as directed by other Proposals or by Dynastic Rules.

Then, add a new Subrule to the Rule entitled “Viral Proposals”.  Call it “Immune System” and give it the following text:

If no Player has previously done so in the current week, any Player may post a Viral proposal.  That Viral Proposal’s Title shall be “[Viral]:The X Purge” where X is the ordinal immediately after the one in the title of the most recent proposal created under this rule, or ‘First’ if there is no such proposal.  The Viral Proposal’s text shall be “Fail all pending Viral proposals other than this one.”