Compressions are more important than breathing as long as hypoxia isn't the cause of the arrest. If it is a prolonged arrest, you can be prepared and carry something like this on your keyring.
http://www.firstaidandsafetyonline.com/files/categories/First%20Aid/CPR%20MDI73-402%20Micromask%20LG.jpg
I've got a couple similar to that which stay in my first aid kit in the back of my car, but not on my person. Some public stores will have these items in the kit with their AED.
But if we think about it, what infectious diseases are we really worried about being transmitted? Barring tongue lac, HIV is very, very unlikely, even oral STDs such as chlamydia are rarely transmitted orally to anything, and to be honest a rare occurrence to be transmitted genitally to orally. HSV-1? who hasn't had a canker sore in their life anyways? HPV? doubt it, Hepatitis? perhaps if they just finished filming a XXX porno. As mentioned, TB, and bacteria which cause pneumonia can be transmitted that way as many are normal colonists of the orapharynx anyways, but that doesn't concern me too much. TB, Influenza, and possibly SARS I guess are probably the worst things I could imagine catching from performing mouth to mouth.