all of us have all kinds of things living in our mouths and they never completely go away. with brushing and flossing removes the majority and help keep our mouths healthy. it similar to yeast. everyone has a little but when you have too much it becomes a problem and if one person has too much it can be passed to another.
Good question - all I know is that the caries causing bacteria streptococcus mutans does get passed on, otherwise parents would not be advised not to share eating utensils, food, drinks, toothbrushes with their infants/ toddlers and not to kiss them on the mouth. Infants do get infected that way but the severity differs. They seem to be especially vulnerable during a period of about six months as infants but I guess a mix of genetics and brushing and flossing techniques will make the difference who gets problems or not later on.
A long long time ago I had a boyfriend with really bad, well, rather sparse brushing habits - and even though I did not develop cavities, I found that my gums were irritated most of the time, regardless of brushing and flossing. I guess whoever you kiss you share the "germs".