There is nothing as romantic as two lovers sharing a kiss.
But scientists have come up with an evolutionary explanation which perhaps threatens to kill the passion.
Academics think that kissing helps partners share bacteria, shoring up their immune systems and enabling them to better fight disease.
As many as 80 million bacteria are transferred during a ten-second kiss, according to Dutch biologists.
Sharing those germs means both partners are equipped to ward off the infections they might introduce to each other later on.
Humans carry trillions of bacteria in the body, which together make up a "microbiota" - a complex mix of bugs which play a crucial role in digesting food and warding off infections.
Remco Kort, from the Netherlands Organisation for Applied Scientific Research - or TNO - said his team set out to discover the evolutionary reason for kissing.
After testing 21 couples, they think kissing helps form a shared microbiota, a similar mix of bacteria living in the body.
He said: "Intimate kissing involving full tongue contact and saliva exchange appears to be a courtship behaviour unique to humans and is common in over 90 per cent of known cultures.
"Interestingly, the current explanations for the function of intimate kissing in humans include an important role for the microbiota present in the oral cavity, although to our knowledge, the exact effects of intimate kissing on the oral microbiota have never been studied.
"We wanted to find out the extent to which partners share their oral microbiota, and it turns out, the more a couple kiss, the more similar they are."
The researchers, whose findings are published in the journal Microbiome, found that couples who share nine intimate kisses a day had a very similar microbiota, meaning they would be better prepared to deal with similar infections and digest similar food.
Scientists have long warned that modern obsession with hygiene and cleanliness has driven a boom in allergies and health problems.
According to the "hygiene hypothesis", increasing prevalence of allergies such as asthma are caused because we are not exposed to enough germs in our daily life.
Professor Graham Rook, an immunologist at University College London, has gone so far as to say that picking food off the floor, buying a dog and regularly kissing your relatives are some of the best ways to ward off allergies.
Speaking at Cheltenham Science Festival earlier this year, he advised that when a baby spits out its dummy, a mother should lick it clean and put it back in the infant's mouth.
He said the problem is that the modern body is at a "constant state of alert" because it is not used to living with germs.
"When the immune system is not needed it should get turned off completely," he said.
"What happens these days is that often it is on a constant state of alert and it is not turned off completely.
"It will do something completely pointless like attacking grass pollen wafting past in the breeze, or attacking the neighbour's cat when it happens to walk past, then you are going to have allergic problems."
- Daily Mail