Vaginal Thrush (Yeast Infection) - Causes

What causes thrush?

Thrush is an infection caused by types of fungal yeast called Candida spp. Small numbers of Candida spp. commonly live on the skin and around the vaginal area. These are usually harmless. The immune system and the harmless germs (bacteria) that also normally live on the skin and in the vagina usually stop Candida spp. from thriving. However, when conditions are good for Candida spp., numbers multiply and may invade the vagina and cause symptoms.

The conditions most liked by Candida spp. are warm, moist, airless parts of the body. This is why the vagina is the most common site for candidal infection. Other areas of the body that are prone to candidal infection include the groin, the mouth and the nappy area in babies.

Most causes of thrush are a result of Candida albicans but sometimes other types of Candida spp. such as Candida glabrata or Candida tropicalis are the cause.

Who gets thrush?

Up to three quarters of all women will have at least one bout of thrush in their lives. In most cases it develops for no apparent reason. However, certain factors can make thrush more likely to develop. The vagina contains mucus and some harmless bacteria which help to defend the vagina from candidal infection (and other germs). These natural defences may be altered or upset by certain situations:

So, in these situations, you may be more likely to develop thrush.

There is some relationship between thrush and the female hormone oestrogen. Thrush is much more common in the "reproductive years", ie the years between starting to have periods and stopping for menopause. Some women are prone to thrush in certain times of their menstrual cycle - for example, before a period. The hormone changes of pregnancy make thrush more likely. It may be that hormone pills such as hormone replacement therapy (HRT) and the combined oral contraceptive (COC) pill make you more prone to thrush. This is not yet certain, however.

What causes recurring thrush?

Some women develop repeated (recurrent) thrush. Recurrent thrush is defined as a bout of thrush four or more times in a year. Of women who develop a first bout of vaginal thrush, about 5 in 100 of them will get problems with recurrent vaginal thrush. In most cases, the reason why this occurs is not known. Some women just seem more prone than usual to develop thrush. However, women with uncontrolled diabetes and women with a poor immune system may be more likely to develop recurrent thrush. There is some debate as to whether women taking HRT or the COC pill are more likely to develop recurrent thrush - the evidence is not yet clear.

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Author:
Dr Mary Harding
Peer Reviewer:
Dr Hannah Gronow
Document ID:
4350 (v43)
Last Checked:
01 July 2017
Next Review:
30 June 2020

Disclaimer: This article is for information only and should not be used for the diagnosis or treatment of medical conditions. Patient Platform Limited has used all reasonable care in compiling the information but make no warranty as to its accuracy. Consult a doctor or other health care professional for diagnosis and treatment of medical conditions. For details see our conditions.