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Having sex without a condom can dramatically increase women's' chances of developing bacterial vaginosis (BV) and other infections. Researchers have found that unprotected sex with a male partner causes disruptions to the delicate balance of bacteria found in the vagina, placing women at higher risk of infections.

Even if women use preventative treatment, sex without a condom can upset the bacterial balance. This is because a vagina tends to have higher levels of a particular strain of bacteria, which works to protect it against infections like BV.

Researchers from Melbourne's Monash University, Australia, have warned that sex without a condom can lead to higher levels of two bacteria that are linked to BV and so cause an imbalance that puts women at risk of painful and recurrent infections.

It was found that this risk is increased even further when it comes to having unprotected sex with a new partner.

All vaginas have five strains of bacteria present, with the most dominant of these usually being Lactobacillus. Many researchers have found that the healthiest vaginas are those where the Lactobacillus crispatus strain is the dominant bacteria.

This bacteria helps to maintain the vagina's pH balance while also fighting off a range of infections caused by bacteria, viruses and yeast. In fact, women whose vaginas have this strain as the most dominant bacteria are better able at fighting off all infections, including sexually transmitted infections (STIs).

However, sex without a condom can reduce the amount of Lactobacillus crispatus bacteria present due to the introduction of other bacteria, as well as semen. This means that infections like BV can start.

Sex without a condom, especially with a new partner, was found by this new study to increase the levels of Gardnerella vaginalis - which has recently been linked to an increase in recurrent urinary tract infections (UTIs) - and Lactobacillus iners.

This means that condoms could be important for vaginal health - especially when having intercourse with numerous or new partners - as well as for stopping the spread of STIs and preventing pregnancy.