Some diseases are contagious, and some are not. The World Health Organization has called dental cavities the most common non-communicable (non-contagious) disease worldwide. But there has been (thanks to social media) some confusion about whether cavities are actually contagious. Can you actually get dental cavities from kissing?
With a Kiss
It's not as though someone's tooth decay can be transplanted to your mouth with a kiss. This is not how the decay and progressive breakdown of a tooth's structure happens. However, the exchange of saliva during a kiss means that you're potentially introducing cariogenic bacteria into your mouth.
Cariogenic bacteria are microorganisms capable of eroding teeth, and all mouths have them. The bacteria interact with certain components of a person's diet (primarily sugars and carbohydrates), and this process creates an acidic byproduct. Over time, the tooth's protective dental enamel is corroded by this acid, and a cavity begins to form. Without treatment, the cavity will generally deepen, penetrating the tooth's dentin (which is usually protected by its enamel), and eventually causing inflammation of the tooth's nerve, which might then require root canal treatment. Does this mean that you should refrain from kissing someone with a lot of cavities?
Managing Cariogenic Bacteria
Just because your partner has tooth decay, it's certainly not inevitable that you will then be affected as well. Many people effectively manage the level of cariogenic bacteria in their mouth by being sensible with their diet (regulating the amount of sugar and carbohydrates they consume), as well as being attentive to their brushing and flossing. Your regular visits to your dentist are also invaluable, as your dentist can remove accumulated plaque and tartar from your teeth (which your own toothbrush cannot). A dentist can also spot early signs of cavity formation, allowing them to strengthen the tooth's enamel with fluoride and other types of remineralization—which can prevent cavities from actually forming.
You don't need to be concerned about catching cavities from kissing, although, if your partner has noticeable tooth decay, you may wish to be mindful of the risk, as minimal as it is. This doesn't require much effort, and you might just wish to get into the habit of rinsing your mouth after kissing, helping you to neutralize any newly-introduced cariogenic bacteria. This can be achieved with an appropriate mouthwash or even a saltwater rinse.
Although cavities themselves are non-contagious, the bacteria that helps to cause them can certainly be transferred from one person to another. This is nothing to be alarmed about, but might require some minor caution (along with perhaps suggesting to your partner that they might be overdue for an appointment with their dentist).
Contact a local general dentist for more information.