So the feat is here:
I'm left with a few questions:
1) can you cover natural weapons with this filth?
2) how about ammunition, like an arrow?
3) I don't see anything saying you can only coat your own weapons with filth. Is it reasonable to say you can help allies by covering THEIR Weapons?
1) Yes; it doesn't specify manufactured weapon, so you can coer natural weapons.
2) Yes; ammunition counts as a weapon, though, since it doesn't make any special call out, you would have to coat 1 piece of ammo at a time.
3) In fact it doesn't seem to even remotely imply that you can only coat your own weapons; you can coat any weapon you like.
Fith fever is an injury vector disease, so I think that isn't much of a danger (well, unless the natural weapon wielder is in the habit of scratching themselves). They may have to keep thier mouth open until thier first attack if it is a bite, though (or risk washing the filth off, yum!).
One might even think that blunt weapons wouldn't be very suitable for this feat, and, yet, no such exception is made, so I guess you can coat your sap or whip with it and knock your foes out while you give them a disease. (you only have to succeed at the attack, not do any damage, so it doesn't matter if it is subdual damage or whether subdual damage counts as damage).
The feat has lots of silly things you can do. :)