Thrush is not considered an STD and it is not that serious. Thrush is more like a yeast infection.
Thrush is the widely used term for a common fungal infection caused by Candida albicans. Usually this takes the form of trivial vaginal or mouth infections, although it can affect the body more widely and seriously on rare occasions.
Vaginal Thrush
This causes itch and a cheesy discharge. The area is often redder than usual. It is often painful too, especially on intercourse, and may be associated with frequency of passing water and burning or pain on passing water. It may, therefore be confused with bladder infections (cystitis). Thrush, however is usually associated with itching, which cystitis is not.
Causes
The cause is a fungus which is found widely in nature (Candida albicans), and indeed, is frequently present in the mouth and motions of people in good health. It seems that it is at periods when the defence mechanisms are down that it actually causes a problem. This can be when physically or mentally "run down", in women either pregnant or on the oral contraceptive pill, in diabetes and people either on steroids, or whose immune systems are suppressed.
Antibiotics, which deplete the naturally occurring bacteria, which are useful to the body, as well as the invading bacteria which they are being used to treat, may disturb the natural balance of the body and lead to thrush developing.
Thrush is otherwise known as Candidiasis, Moniliasis, or yeast infection.
Treatment
Locally applied creams, pessaries, lozenges or gels, depending on the site affected. The most widely used are the imidazoles although there are other useful preparations. If in doubt consult with your doctor or pharmacist.
Systemically administered preparations. These are costly, but very effective, and usually only require a single dose.
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