Does saliva swapping/transfer have any affect on dental health?

If you shared food with somebody who had plaque, gum disease, gingivitis -would you have a high chance of getting it or an increase risk?

Both tooth decay and periodontal disease are communicable.

I have had a couple of cases in my 30 year practice where I observed this directly. In one, the husband had really good teeth. He got married to a lady who had horrible periodontal disease. She had already lost some of her teeth by the time they got married. (I guess he didn't look in there first.) It took a couple of years, but he started to have nasty gum infections. In spite of coming in every 3 months for cleanings once this pattern was observed, he lost a few teeth to gum disease. By the time they got divorced, he had lost 4 or 5 teeth. After the first 3-month recall (cleaning) visit, we moved him out to 6 months and he has been doing just fine at the old standard 6 month interval. When he was married and swapping spit with his wife, he just could not overcome the bad germs that he was picking up from her, no matter how good his home care or office care was. When he left her, her germs left him and he has been fine. This particular case played out over almost all 30 years of my practice so I got to see him a couple of years before they were married, all 12-15 years of there marriage, and at least 10 years since their divorce. I'm convinced.

any time someone has issues with their mouth, best not to swap spit.

yes, many different diseases can be transmitted just by saliva.