Sexually transmitted infections (STIs) are a group of infections similar to one another only in that they can be acquired through sexual contact. STIs is a term now used in place of sexually transmitted diseases (STDs).

You don't necessarily have to have sex to get sexually transmitted infections (STIs), but sexual activity is the most common way for them to be spread. The infections are caused by different organisms and have a wide variety of symptoms.

Here are some of the most common STIs:

• Chlamydia occurs most often in teens and young adults, with females much more likely to get it than males. It is caused by the bacteria Chlamydia trachomatis. In Canada, chlamydia is the most common STI.
• Gonorrhea is an infection by the bacteria Neisseria gonorrhea. It is the second-most common STI, and teens and young adults represent almost half of all cases that are reported. If this condition is not treated, it may lead to infertility in both men and women.
• Human papillomavirus (HPV) causes genital warts. It is also a very common STI in Canada, especially among teens and young adults. Research shows that the virus that causes genital warts is also linked to cervical cancer, as well as certain other cancers including cancer of the anus, penis, vulva, vagina and the back of the throat.
• Syphilis is a bacterial infection that is less common but still remains a cause for concern. Its frequency has dropped significantly over several few decades until the late 1990s, after which the rate has been steadily increasing in Canada.
• Human immunodeficiency virus (HIV) is the viral infection that can cause AIDS (acquired immune deficiency syndrome). This virus attacks cells of the immune system, leaving a person defenceless against many other infections and their complications.
• Hepatitis B is caused by a virus that infects the liver. There is no cure for this condition, but a vaccine is available to protect you against the virus.
• Genital herpes, which produces cold-sore-type lesions, is also caused by a virus. Once the herpes virus enters your body it is there for the rest of your life. Symptoms of the infection may occur without warning.
• Chancroid, a bacterial infection of the genitals, has been considered to be rare in North America, but the true number of cases may be difficult to determine since it is hard to test for. It can cause genital ulcers.
• Pubic lice, also known as "crabs," is an infestation of the genital area by lice (tiny wingless insects).
• Trichomoniasis is an infection caused by Trichomoniasis vaginalis, a protozoan that affects women more often than men, and is often asymptomatic. This infection commonly appears along with gonorrhea and other STIs.

STIs are usually caused by bacteria or viruses. Your chances of getting them are highest if:

• you have unprotected sex
• a condom tears during intercourse
• your sexual partner has an STI
• your partner is engaging in sex with other people

In some cases, such as with HIV or hepatitis B, viruses can also be spread through infected blood or shared needles and syringes, including those used for drugs, body piercing, or tattoos. Pregnant women can pass certain infections (e.g., HIV) on to their babies prior to or during birth, or while breast-feeding.

Growth of genital warts caused by HPV may appear during pregnancy because of changes in hormone levels.

Many people with STIs might have no obvious symptoms at all. As a result, the person may not seek treatment for a long time. This delay could result in higher risks of STI-related health problems or complications, as well as the possibility of spreading the STI to partners.

A number of symptoms can indicate the existence of an STI, although specific symptoms are unique for different infections:

• heavier than normal vaginal discharge
• discharge from the penis or rectum
• itching in genital or anal areas
• sores or rashes in genital or anal areas, sometimes also in the mouth
• pain during intercourse
• painful or more frequent urination
• swollen glands in the groin
• fever, headache, general feeling of illness
• pelvic pain that is not related to your period

With syphilis, sores called chancres often appear about 3 weeks after exposure. There are usually one or more sores at the place of initial infection. If left untreated, this first phase of syphilis lasts 3 to 6 weeks. A rash over larger areas of the body can follow weeks to months after the sores appear. This is the beginning of the second stage of syphilis. People with syphilis may also get aching muscles and swollen lymph glands as well as flat warts during this stage. Syphilis can also lead to eye inflammation, causing blurred vision. In the second stage, symptoms may come and go over the next 1 to 2 years, then disappear, becoming latent syphilis. About one-third of people in the second stage of syphilis will go on to the third stage, also known as late syphilis, where the infection damages the brain, heart, nervous system, bones, joints, eyes, and other body areas. The third stage may appear any time from 1 to 30 years after the first infection.

Hepatitis B can cause many symptoms including a decrease in appetite (associated with nausea and vomiting), jaundice, dark yellowing of urine, and aching in the muscles and joints. These symptoms are signs of liver inflammation or damage.

Genital herpes produces a tingling sensation in the genitals. Sores develop in and around the male and female genitals, anus, thighs, buttocks, and mouth.

Chancroid is caused by a bacteria infection in the genital area. Around 4 to 7 days after exposure to the bacteria, sores form, often with a red border around them. Although this infection is more common in tropical areas, it is possible to get it elsewhere. Antibiotics treat this infection normally within 2 weeks.

It's possible to transmit pubic lice from one person to another without sexual contact (for example, by sharing bedding, towels, or clothing). However, sexual contact may transfer the eggs or lice from one person to another. Symptoms may include itching of the genital area. You may also be able to see the lice (small, brown, pinhead-sized insects) or their eggs (oval and whitish in colour) in your pubic hair. Wash clothes and bedding in hot water if you discover pubic lice and speak to your doctor or pharmacist for ways to treat the problem. Medicated shampoos or rinses are available over-the-counter to treat pubic lice, which usually requires more than one treatment. 

There are serious complications associated with many of the STIs:

• Infertility, pregnancy complications, or higher risks of cervical cancer can occur in women.
• Gonorrhea, if not treated, can spread via the blood stream to joints and heart valves.
• Both gonorrhea and chlamydia can cause eye infections in newborns that came in contact with the bacteria during delivery.
• If syphilis is not treated, it may eventually cause serious damage to the bones, heart, eyes, brain, and nervous system.
• Hepatitis B can lead to long-term liver damage and higher risks of developing liver cancer.
• HIV weakens a person's immune system, putting them at risk for many different infections.
• Chancroid makes a person more susceptible to HIV infection when they're exposed to the virus.
• An active herpes infection at the end of a pregnancy will require delivery by a caesarean section to avoid spreading the infection to the baby.

If you have some of the symptoms described in this article, or think you may have a sexually transmitted infection, you'll need a medical exam to diagnose and determine the best treatment for the specific STI. Your doctor will ask you questions about your symptoms and perform a physical exam. To help diagnose STI, your doctor may order blood tests, urine tests, or may take a swab from the genital area, which will be sent to a laboratory for evaluation.

Sexually active individuals, particularly those with multiple partners, are recommended to have regular checkups with a family doctor. In some cases, there are no obvious symptoms and the infections that cause STIs can only be identified through regular STI screening tests.

Antibiotics can be used to treat bacterial infections, like the ones that cause gonorrhea, syphilis, or chancroid. Gonorrhea often occurs at the same time as chlamydia, so doctors usually prescribe antibiotics to treat both gonorrhea and chlamydia. Your partner should be treated at the same time to avoid reinfection.

People with acute hepatitis B are usually treated only for symptoms. Most adults clear the virus on their own. However, for the few people that do not, treatment exists to reduce the risk of long-term liver damage.

There is no cure for HIV. Doctors prescribe different combinations of antiviral medications to slow down the progress of the disease. Treatments can vary from one person to the next to determine what combination works best for you. Doctors also treat secondary infections that result from a weakened immune system.

Don't be shy about asking new sexual partners if they have STIs, or letting them know if you have one. To avoid spreading STIs, people who are sexually active and have multiple partners can be routinely screened – and rapidly treated – by a doctor.

If you want to prevent getting STIs, you should:

• Avoid having unprotected sex. Always use condoms and learn how to use them correctly to best protect against STIs.
• Avoid using shared, non-sterile needles for drugs, body piercing, or tattoos.
• Visit your doctor regularly to check for STIs.
• Learn more about STIs. The more you know about STIs, the better you can protect yourself against them. Speak to your doctor or pharmacist and look for resources in your community.
• Stay updated on your vaccinations, including the hepatitis A, hepatitis B, and HPV vaccines.