Pelvic inflammatory disease (PID) is caused by an infection developing in the female reproductive system.

In most cases, the condition is caused by a bacterial infection spreading from the vagina or cervix (entrance to the womb) into the womb, fallopian tubes and ovaries.

PID is often caused by more than one type of bacterium and it can sometimes be difficult for doctors to pinpoint which are responsible.

This means a combination of antibiotics will be prescribed so a variety of bacteria can be treated.

The main cause of PID is through a sexually transmitted infection (STI) such as chlamydia, gonorrhoea or mycoplasma genitalium.

These bacteria usually only infect the cervix, where they can be easily treated with antibiotics. 

But if they're not treated there's a risk the bacteria could travel into the female reproductive organs.

If you have chlamydia and it's left untreated, it may develop into PID within a year.

In many cases, the cause of the infection that leads to PID is unknown.

Sometimes, the usually harmless bacteria found in the vagina can get past the cervix and into the reproductive organs.

Although harmless in the vagina, these types of bacteria can cause infection in other parts of the body.

This is most likely to happen if:

• you have had PID before
• there's been damage to the cervix following childbirth or a miscarriage
• you have a procedure that involves opening the cervix (such as an abortion, inspection of the womb, or insertion of an intrauterine device (IUD)

If an infection spreads upwards from the vagina and cervix, it can cause inflammation of the:

• womb lining (endometrium)
• fallopian tubes
• tissue around the womb
• ovaries
• lining of the inside of the abdomen (peritoneum)

Pockets of infected fluid called abscesses can also develop in the ovaries and fallopian tubes.

Anyone with female reproductive organs can get PID, but you're more likely to get it if you:

• have more than 1 sexual partner
• have a new sexual partner
• have a history of STIs
• have had PID in the past
• are under 25
• started having sex at a young age