The XYZ Affair was a political and diplomatic episode in 1797 and 1798, early in the presidency of John Adams, involving a confrontation between the United States and Republican France that led to the Quasi-War. The name derives from the substitution of the letters X, Y, and Z for the names of French diplomats Jean-Conrad Hottinguer (X), Pierre Bellamy (Y), and Lucien Hauteval (Z) in documents released by the Adams administration.
What was the "X.Y.Z. fever"?
The "X.Y.Z. fevor" is a reference to a diplomatic incident that occurred during the administration of the second President of the United Status, John Adams.   Adams sent a diplomatic delegation to France to negotiate a resolution of issues that were threatening to lead to war.  The French foreign minister, Charles-Maurice de Talleyrand-Périgord (Talleyrand) demanded a bribe and a loan to begin the negotiations.   President Adams refused Tallyrand's request and reported the solicitation to Congress.  The Senate printed President Adam's report which referred to the three French diplomats involved, Jean-Conrad Hottinguer, Pierre Bellamy, and Lucien Hauteval as "X,Y, and Z" respectively.   
The populace supported Adams decision to refuse Tallyrand's bribe request.  Vice-President Thomas Jefferson referred to the outpouring of support for Adam's political party that followed  as "X.Y.Z. fevor".