There are many factors that can encourage yeast to multiply and cause thrush:
-warmth and moisture
....Yeasts flourish in a warm, moist enviroment. So wearing tights, nylon pants or tight jeans will encourage the yeast to grow
.... during pregnancy the increased level of oestrogenen cause a change in the body tissue which allows yeasts to grow more easily. So thrush infections are more common during pregnancy
....they do not affect yeasts directly, but they kill other organisms in the body leaving the yeasts more room to muliply and cause an infection. So thrush can flare up after a course of antibiotics.
.... although a thrush infection is often caused by multiplication of yeasts that are already there, it is also possible to catch thrush from someone else.