Is that helpful? The problem is that people who bite their nails can not help it, so suggesting that they are putting poo in their mouths is not really the point is it! And it's not going to help a nail biter telling them that. It'll only make a nail botherer more likely to bother their nails in fact.
a - get a manicure
b - spray some spray plaster on the bothered bits.
c - put loads of hand cream on
d - don't beat yourself up about it
e - it'll go away eventually if you 'let it go' ie don't beat yourself up about it!
I'm an oldie on here so have a few years on most people so I know and I've been battling nail bothering for longer than I can care to mention. It goes in phases with me normally when I'm worried or stressed or maybe just that the weather is making my skin really dry and then I pick at my fingers. The habit can make me feel utterly miserable especially when it goes into a kind of obsessive compulsive direction. But the above has helped me enormously. I've been so embarassed having a manicure with nails like they've been. However manicurists are very sympathetic and with a bit of tlc, they grow back.
Don't be tempted to put acrylic false nails on because it makes the problem worse and damages the original nails.
Although mentioning OCD, I actually have diagnosed OCD so that's probably why the thought of putting crap in my gob, along with studying biomed, stopped my nail biting (which was excessive due to OCD anxiety).
Half the time when biting your nails you are consciously unaware you're even doing so! So if in your mind you are perpetually nagging yourself not to bite your nails, this should help you to overcome the habit completely. - I know it worked for me!