Many cases of STIs can be asymptomatic or have only mild symptoms that may not be recognised as an STI. However, symptoms commonly indicating STI include vaginal discharge, urethral discharge accompanied with burning pain (in males and females), genital ulcers, and abdominal pain.1,7 

More serious outcomes include: 

• in women, pelvic inflammatory disease (PID) with increased risk of infertility or ectopic pregnancy due to scarring in the fallopian tubes and ovaries8
• in men, swollen and painful testes (epididymo-orchitis/epididymitis) and possibly infertility.9 

If untreated, longer term problems can include increased risk of:1,2,8,9,10,11,12 

• septicaemia (eg with gonorrhoea)
• sexually acquired reactive arthropathy (SARA) when joints, especially in the hips, legs or feet become infected causing reactive arthritis
• cervical and other cancers from HPV
• conjunctivitis
• prostatitis (eg with gonorrhoea, chlamydia or HIV)
• damage to the nervous system (central and peripheral) and cardiac problems with syphilis.  

Mother-to-child STI transmission can result in stillbirth, neonatal death, prematurity and low birth weight, congenital deformities, or infection in the child such as sepsis, pneumonia or neonatal  
Chlamydia and gonorrhoea are the main STIs associated with PID. However, non-STI causes can be a bacterial infection spreading from elsewhere in the abdomen, such as acute appendicitis, or occasionally due to surgical abortion.13 
A much less common cause is being fitted with an intrauterine device (IUD or coil) with the PID developing within three weeks of the IUD insertion.However, this risk can be significantly reduced if the woman is tested and treated for any infection or STI before the fitting.