Jean-Paul Marat's skin disease worsened slightly faster than OTL, and his doctors insisted that he go on a medical retreat from unsanitary Paris.
A prideful man, he only agreed to go after he was given a commission to inspect the process of the Republc's armies as they crossed into Germany. Conveniently, this meant that he could 'inspect' the supply depot at Bad Ems. He bathed regularly both at the springs and in the nearby river, though this did little to soothe the irritation.
In the meantime, his absence from Paris meant that the Girondins proved more secure when the National Convention turned against them, and instead of ten members of the Commission of Twelve being arrested only a single Deputy was removed from office.
Hence, it was later said of Marat's life that An Itch in Rhine Saved Nine.
I have a problem, I know, but I spent far too long trying to work all possible variations on the phrase and only then did I realise I needed to fit some alt-history to it...