Intimate kissing of many different partners can quadruple a teenager's risk of meningitis, a study has found.
Kissing one person was not linked to increased risk
Kissing with tongues enables the potentially deadly meningococcal bacteria to pass between partners.
The Australian team which carried out the British Medical Journal study of 144 teenagers defined multiple partners as between two and seven in two weeks.
Lead researcher Robert Booy said teenagers should change their behaviour - but accepted most would not.
Meningococcal disease is a life threatening condition.
Incidence tends to peak in early childhood and in adolescence.
It can cause meningitis, an inflammation of the brain lining, or meninges, and septicaemia, which is the blood poisoning form of the disease.
The incidence and fatality rate among teenagers in England and the United States rose dramatically during the 1990s.
The introduction of the meningitis C vaccine in the UK in late 1999 - which is given to both babies and teenagers - has helped numbers drop but other forms of the infection remain a major problem.
It is known that around one in 10 teens carry meningococcal bacteria.
The researchers questioned 144 teenagers aged 15 to 19 diagnosed with meningitis at English hospitals.
Each was then compared with another teenager of the same age from their GP's list.
The research team asked about factors which might increase or decrease the teenagers' risk of meningitis.
SOME COMMON SYMPTOMS
High temperature, fever, possibly with cold hands and feet
Vomiting, sometimes diarrhoea
Joint and muscle pains, possible stomach cramps
Dislike of bright lights, disorientation
Blood samples and nose and throat swabs were also taken.
In addition to kissing multiple partners, a history of preceding illness, and being a student were linked to an increased risk of disease, while attendance at a religious event was linked to a lower risk.
Other factors can increase someone's risk of becoming ill if they are exposed to the bacteria, the researchers found.
Having had the Epstein-Barr virus - itself called the 'kissing disease' - is a factor, as is having an upper respiratory tract infection. Being a student also appeared to increase risk.
But attending religious services did not.
The researchers suggest these last two factors could be 'markers' of behaviour, indicating whether they are more likely to have multiple kissing partners and be attending parties, where the meningococcal bacteria can also be passed on by smokers' coughing.
Being born prematurely was also a risk - appearing to affect immunity to infection even 15 to 20 years later.
Teenagers 'not impervious'
Professor Robert Booy, co-director of the National Centre for Immunisation Research and Surveillance at the Children's Hospital Westmead, Sydney, led the study.
He said: "We all carry bacteria in out throats, and some of us will carry the meningococcal bacteria.
"They don't exist very well outside of the body. But if it can pass in saliva, from one person to the next, then it passes very easily."
He said the findings might make teenagers think.
"They tend to think they are impervious and will live forever. But if the message is this is something that could kill them in three days, it's a little more adjacent."
Professor Booy said he accepted that teenagers would continue to behave as they always had.
But he added: "I think they should change their behaviour. We found the risk for those without a partner and those with one was the same.
"The message to people who snog two partners is that they've probably also snogged two people, so you are multiplying your risk."
Linda Glennie, head of research at the Meningitis Research Foundation, which funded the study, said: "I don't expect teenagers to become nuns and monks for the duration of their university career, but I would encourage them to be aware of the symptoms."
Philip Kirby, chief executive of the Meningitis Trust added: "This research helps to highlight why students are the second most 'at risk' group for meningococcal disease.
"In the general population about 10% of people carry the meningococcal bacteria in the back of their throats, but in the student age group we know that this can increase to 30% due to their increased social interaction.
"The study reinforces the need for continued awareness amongst 15 to19-year-olds."
A Department of Health spokeswoman said: "Our advice already highlights kissing as a risk factor for meningitis.
"We keep all new research under review and always amend our advice if necessary."