Well you should condoms, which is sort of obvious, to drastically reduce the risk of exposure. In regards to something contagious in their mouth, it is actually not really good to be too clean because it leads to compromised immunity. Read this - very helpful. Dirty kids are healthy kids - the Hygiene Hypothesis Worked very well for me - as soon as I stopped using hand santizer and started limiting hand washing to when it is really essential, I stopped getting respiratory infections, and in the past two years was sick only once for a few days and that following a transatlantic flight that probably made me prone to getting sick with a virus.

And I share food regularly by eating from the same plate in a restaurant with people and never ever get any sort of GI upset. The only time I ever had any kind of problem with the stomach discomfort was when I was trying Topamax (which was altogether disastrous so I stopped).

I used to have an Indian colleague who did the same without knowing of the hygiene hypothesis (that the western world is too clean) and just because she grew up in dirty India and believed that she had immunity to all diseases. But, to tell you the truth, she never got sick either.

So all of that preoccupation with cleanliness is not healthy. Sure, testing/condoms for sex is important, but preoccupation with being too clean in general is not good.