A normal healthy vagina is slightly acidic because it contains 'good' bacteria, called lactobacilli, which produce lactic acid that prevents 'bad' bacteria from growing. If this balance of bacteria changes the levels of acidity reduces which can cause BV.

pH balance

pH is the scale used to measure the acidity or alkalinity of a solution. Most solutions appear on the scale between 0 and 14 with 7 being classified as neutral. Anything below 7 is an acid and above is an alkaline. The vagina has a normal pH balance of 4-4.5 anything that changes this balance can enable fungi or bacteria to multiply. If a woman has BV the pH balance will be greater than 4.5 which make the vagina more favourable for the growth of specific types of bacteria.

The reason for the changes in acidity is unknown but there are some factors which can increase a woman's risk of developing BV:

• Recent change in or multiple sexual partners
• Participation in certain sexual activities
• Vaginal douching or bubble baths
• Use of a copper intra-uterine device
• Menstruation or the presence of semen can increase the pH balance
• Smoking, this is thought to be due to a reduction of lactic acid producing bacteria

BV is not recognised as a sexually transmitted disease as there is no equivalent condition for men and whilst it is more common in sexually active women it can occur in those who are not. There is evidence to show that women can pass the infection on to female sexual partners.

There are a few factors which can reduce the risk of developing BV:

• Taking the combined oral contraceptive pill
• Using condoms