Thrush (also called “candida”) is usually caused by the yeast Candida albicans. This yeast lives harmlessly on the skin and in the mouth, gut and vagina. Normally it is kept under control, occasionally conditions change and signs and symptoms can develop.
Conditions that make thrush more likely:
As it lives on the skin it is not sexually transmitted.
Men / I have a penis: red itchy blotchy skin usually on the glans penis (“ bell end”) or under the foreskin, swelling of the foreskin sometimes worse after sex, thicker discharge under the foreskin.
Women / I have a vagina: itching soreness and redness of the vulva, vaginal discharge which is often thicker and can smell “yeasty”, pain when passing urine or having sex.
Bottoms: itching and redness around the bottom.
This is usually diagnosed by appearance (so by looking at the skin) although occasionally the doctor or nurse who sees you may take a sample from the skin on the penis or from the vagina to look at either under the microscope OR to send to the lab to identify the yeast.
Try to avoid using soaps and showers gels/feminine washes on the genital skin - use emollients instead.
Thrush is treated with creams if the skin is irritated and/or oral treatment if there is a vaginal discharge or severe infestation (in men); pessaries (vaginal tablets) are also an alternative (as the oral treatment is not licensed in pregnant women).
It is not a sexually transmitted infection so partners do not need treatment; your partner may have symptoms but this is likely due to them using soaps and shower gels and causing thrush to irritate their skin too.