A sexually transmitted infection (STI) is an infection that spreads mainly through sexual contact. STIs are caused by bacteria, viruses or tiny parasites. Common ones include chlamydia, gonorrhoea, genital herpes, genital warts (HPV), syphilis, HIV, hepatitis B, trichomoniasis and Mycoplasma genitalium.

STIs are common, and anyone who is sexually active can get one. Many are easy to treat, and the viral ones can usually be managed well. The key point is that a lot of STIs cause no symptoms, so you can have one — and pass it on — without knowing. That is why regular testing matters.

Most STIs spread during vaginal, anal or oral sex with someone who has the infection. Some, such as herpes and genital warts, can also spread through close skin-to-skin contact. A few can pass from a parent to a baby during pregnancy or birth, and some can spread by sharing needles. Some infections spread even when there are no symptoms.

Each STI is different, and many cause no symptoms at all. When symptoms do appear, they can include:

• Unusual discharge from the vagina, penis or anus
• Pain or burning when you pass urine
• Sores, blisters, ulcers, lumps or warts around the genitals or anus
• Rashes, itching or tingling
• Bleeding between periods or after sex
• Pain during sex or pain low in the abdomen

Because many STIs have no obvious symptoms, you should have regular sexual health check-ups if you have new or multiple partners. Genital symptoms are not always caused by an STI, but it is worth getting them checked.

If they are not found and treated, some STIs can lead to longer-term problems. These include pelvic inflammatory disease (PID), ongoing pelvic pain, reduced fertility, and a higher chance of an ectopic pregnancy (a pregnancy that grows outside the uterus).

You cannot tell whether you have an STI just from how you feel — a test is the only way to be sure. STI testing is routine, confidential and nothing to feel embarrassed about. Doctors, sexual health clinics and family planning clinics do these tests every day. You can be tested by your GP or a sexual health clinic, and Dr Mahadik can arrange or review testing as part of your gynaecological care.

It is a good idea to get tested, even without symptoms, if:

• You have had sex without a condom with a partner whose STI status you do not know
• You have a new or casual partner
• You think a partner may have had other partners
• A partner tells you they have an STI
• You have had any unwanted or non-consensual sexual contact
• You are planning a pregnancy, so any infection can be treated first
• You have had sex in a place where HIV or other STIs are common

Testing is simpler than many people expect. Depending on the infection, a test may use:

• A urine sample
• A self-collected or clinician-collected swab (vaginal, cervical, throat or anal)
• A blood test (for infections such as HIV, syphilis and hepatitis B)

Your doctor will suggest which tests you need based on your history. Some results are ready quickly; others take a few days.

How often you should test depends on your own situation and level of risk. A yearly sexual health check is a good baseline if you are sexually active, and more often if you have new or multiple partners. It is worth testing with each new partner, even if you always use condoms, because condoms lower the risk but do not remove it completely.

If a test comes back positive, your doctor will explain what it means, what treatment you need, and what to do about recent partners. Most STIs are straightforward to treat, and the viral ones can be managed. Try not to panic — a diagnosis is the first step to getting it sorted.

If you are diagnosed with an STI, recent sexual partners will usually need to be told so they can be tested and treated. Which partners, and how far back, depends on the infection — your doctor will guide you. This can feel awkward, but most people would rather know so they can look after their own health and avoid passing it on. You can let partners know anonymously through a sexual health clinic or an online notification service if you prefer.

Treatment depends on the type of STI:

• Bacterial STIs (such as chlamydia, gonorrhoea and syphilis) can usually be cured with antibiotics — tablets, or an injection for some infections.
• Parasitic STIs (such as trichomoniasis) are also treated with medicine.
• Viral STIs (such as herpes, HIV, hepatitis B and HPV) cannot usually be cured, but antiviral and other medicines control the virus, ease symptoms and lower the chance of passing it on.

It is important to finish the full course of any treatment, avoid sex until your doctor says it is safe, and go back for a repeat test if one is advised. Do not start or stop any treatment without medical advice.

This section describes the more common STIs, how they are tested for, and how they are treated in Australia. It is general information — your doctor will advise on what is right for you.

Chlamydia is one of the most common STIs and is caused by bacteria. Most people have no symptoms, so many do not know they have it. Younger people are more likely to get chlamydia. Left untreated, it can spread to the uterus and fallopian tubes and cause pelvic inflammatory disease (PID). PID can lead to ongoing pelvic pain, a higher chance of ectopic pregnancy, and reduced fertility. A baby can also pick up an eye or chest infection during birth.

When there are symptoms, they can include unusual vaginal discharge, burning when passing urine, bleeding between periods or after sex, lower abdominal pain, or pain during sex.

Chlamydia is usually diagnosed with a urine test or a swab (which can often be self-collected) from the vagina, cervix or anus.

Chlamydia is treated with antibiotics. In Australia the recommended first-line treatment is now doxycycline, taken twice a day for 7 days, because it works better than a single dose, especially for infection in the anus. A single dose of azithromycin may be used in some situations — for example, in pregnancy or if finishing a week's course would be difficult. You should avoid sex until 7 days after treatment starts, and until any partners have been treated. A repeat test is usually advised about 3 months later to check for reinfection.

Yes. Having had chlamydia does not protect you from getting it again.

Gonorrhoea is caused by bacteria. It usually affects the genitals but can also infect the throat or anus. It spreads through vaginal, oral or anal sex. Like chlamydia, it often causes no symptoms, so it can be passed on without you knowing. Untreated, it can also cause PID.

When there are symptoms, they can include unusual vaginal discharge, pain when passing urine, pain during sex, or a sore throat if the throat is infected.

Gonorrhoea is diagnosed with a urine test or a swab from the cervix, throat or anus.

Gonorrhoea is treated with antibiotics. In Australia the standard treatment is a ceftriaxone injection together with a dose of azithromycin taken by mouth. Two medicines are used together because some strains of gonorrhoea have become resistant to antibiotics. This is more likely with strains picked up overseas, so tell your doctor if you have travelled. A follow-up test is usually advised about 2 weeks later to check the treatment worked, and again at 3 months to check for reinfection. Avoid sex, even with a condom, until your doctor says it is clear.

Genital herpes is a common STI caused by the herpes simplex virus (HSV). Around 1 in 8 sexually active Australian adults have it. There are two types of the virus. HSV-1 more often causes cold sores around the mouth, but it can spread to the genitals through oral sex. HSV-2 is more often found around the genitals. Herpes spreads through skin-to-skin contact during vaginal, oral or anal sex. Most people with herpes have no symptoms, or such mild ones that they do not notice, which is part of why it is so common.

Herpes tends to come in "episodes", and the first one is often the most uncomfortable. It can bring flu-like feelings, such as a headache, aches, swollen glands and feeling generally unwell. There may also be tingling or itching, redness, and small blisters that break open into painful sores, sometimes with pain when passing urine. You should not have sexual contact when you have visible sores, even with a condom.

A doctor takes a swab from an affected area to confirm herpes. A blood test is sometimes used as well.

There is no cure for herpes, but antiviral medicine can ease symptoms, make episodes shorter and less frequent, and lower the chance of passing it on. For symptoms during an episode, it can help to take simple pain relief, use a numbing cream, and bathe the area in warm salt water. Speak to your doctor about what is right for you.

Episodes usually become milder, shorter and less frequent over time, and for many people they eventually stop. Episodes may be triggered by stress, illness, lower immunity, periods or sex. Herpes is most easily passed on during an episode — when there is a sore, blister or split in the skin — so avoiding sexual contact at those times lowers the risk.

Herpes does not affect your ability to get pregnant. In pregnancy it needs some planning, because the virus can rarely pass to a baby around the time of birth. Tell your doctor or midwife if you or a partner have ever had genital herpes, so your care can be planned. If you are planning a pregnancy or already pregnant, Dr Mahadik can talk this through as part of your antenatal care.

Syphilis is caused by bacteria and spreads through close skin-to-skin contact during sex, and to a baby during pregnancy. It is very contagious when sores or a rash are present. Syphilis is less common in Australia than chlamydia or gonorrhoea, but cases have been rising. It can cause serious health problems if left untreated, but it responds well to treatment, especially when found early.

Syphilis develops in stages, and the signs differ at each stage:

• First stage: one or more painless sores, often 3 to 12 weeks after contact. Sores can be easy to miss, especially if they are inside the vagina or mouth, and they heal on their own — but the infection is still there.
• Second stage: a rash (often on the palms or soles), swollen glands, hair loss, joint pain or flu-like feelings. This stage is contagious.
• Later stage: if still untreated after many years, syphilis can cause serious heart or brain problems. It is not contagious at this point but is still serious — and still treatable.

Syphilis is usually diagnosed with a blood test, and sometimes a swab of a sore.

Syphilis is treated with an injection of a long-acting penicillin. If you are allergic to penicillin, an antibiotic such as doxycycline can be used instead. Early treatment stops the infection progressing and lowers the risk to others. Avoid sexual contact until your doctor confirms it is safe, and attend follow-up blood tests so your treatment can be checked.

Genital warts are small lumps or bumps around the genitals or anus, caused by some types of the human papillomavirus (HPV). They spread through skin-to-skin contact during sex. A doctor can usually diagnose warts by looking at them. They often clear on their own as the body deals with the virus, but treatments — such as creams or freezing — can remove them sooner. The HPV vaccine protects against the types that cause most genital warts and most cervical cancers, which is why it is offered to young people in Australia. HPV is also the reason for regular cervical screening.

HIV (human immunodeficiency virus) affects the immune system and spreads through blood, semen, vaginal fluids and breast milk — usually during sex without a condom or by sharing needles. HIV is diagnosed with a blood test. There is no cure, but modern treatment is very effective: it keeps people well and, when the virus is fully controlled, means it cannot be passed on to sexual partners. Medicines can also lower the risk of catching HIV — PrEP (taken before possible exposure) and PEP (started within 72 hours after possible exposure). Your doctor or a sexual health clinic can advise.

Hepatitis B is a virus that affects the liver. It can spread through sex and through blood, and from a parent to a baby around birth. Many people clear the virus, but some carry it long term, which can affect the liver over time. Hepatitis B is diagnosed with a blood test. There is a safe, effective vaccine, which is part of the routine childhood immunisation schedule in Australia and is also offered to adults at higher risk. Long-term infection can be monitored and managed with medicine.

Trichomoniasis ("trich") is caused by a tiny parasite and spreads through sex. It can cause an unusual, sometimes frothy discharge, itching, soreness and discomfort when passing urine — but often there are no symptoms. It is diagnosed with a swab or urine test and is easily treated with antibiotics. Recent partners should be treated at the same time to avoid passing it back and forth.

Mycoplasma genitalium (often shortened to "MG") is a bacterial infection that spreads through sex. Most people have no symptoms, but it can cause discharge, irritation or, less often, PID. It is diagnosed with a urine test or swab, usually done when there are symptoms. MG is treated with antibiotics, though some strains have become resistant, so treatment is sometimes given in two steps and a follow-up test may be advised.

You cannot remove the risk of an STI completely, but a few things lower it a lot.

Condoms (external and internal) are the only contraception that also lowers the risk of many STIs. Using condoms with new partners is one of the simplest ways to protect yourself, and they can be used alongside another contraceptive method. Condoms do not cover every part of the skin, so they lower the risk of STIs like herpes and warts without removing it.

Vaccines protect against two STIs. The HPV vaccine protects against the types of HPV that cause most genital warts and most cervical cancers, and is offered to young people in Australia. The hepatitis B vaccine is part of the routine childhood immunisation schedule and is also offered to adults at higher risk. Your doctor can check whether you are up to date.

Many STIs cause no symptoms, so regular sexual health checks help catch an infection early. Aim for a check once a year if you are sexually active, and with each new partner. Testing early means any infection can be treated before it causes problems or is passed on.