When two people kiss, they exchange bacteria and viruses in their saliva. Fortunately, most of these bacteria are not dangerous, but instead can work as a kind of oral vaccination and stimulate the immune system when transferred from one partner to another. Bacteria that cause tooth decay and mouth odour will not survive if they move to a new site. Viruses are a different matter. Glandular fever, or mononucleosis, caused by the Epstein-Barr virus is rightly called the kissing disease. This virus attacks the mucous membranes of the nose, mouth and throat and can be transmitted by kissing or through coughs and sneezes. However, most adults have already had the infection-often without realizing it-and are therefore immune. The herpes virus, which causes troublesome sores and blisters around the mouth, is also transmitted by kissing.