Chlamydia is the most diagnosed STI in the UK. It accounts for just under half of all new STI diagnoses in the UK, with around 200,000 cases recorded each year.

Many people tend to presume that STIs such as chlamydia can only be passed on through penetrative sex; however, this is not the case. As we’ll discuss, there are several ways chlamydia can be transmitted between two people, including:

  • Unprotected vaginal, anal or oral sex
  • Sharing sex toys
  • Genital on genital contact
  • Infected sperm coming into contact with the eye

It is also possible for the infection to be transmitted from mother to baby during childbirth.

Chlamydia does not always produce symptoms in the carrier. As discussed on our chlamydia symptoms page, around 7 in 10 women and 5 in 10 men will develop no noticeable signs.

Just because someone does not display any symptoms, however, does not make the condition any less infectious.


Chlamydia trachomatis

There are in fact several different species of chlamydia bacteria. The sexually transmitted disease commonly known as chlamydia is caused by C. trachomatis.

In an infected person, this is present in the mucous membranes at the infected site. Mucous membrane is soft tissue in the body which is not protected by skin, such as the inside of the mouth, vagina, urethra and the rectum.

Mucous membranes emit fluid which can carry C. trachomatis, and it can also be present in semen; meaning that when someone who isn’t infected comes into contact either directly with the mucous membrane, sperm or vaginal fluid of someone who is infected, it is possible for them to pick up the bacteria and develop chlamydia.

Transmission routes

Consequently, the infection can travel from one person to another via the following routes:

  • Unprotected vaginal sex

Chlamydia can pass from male-to-female through infected sperm, or from female-to-male through vaginal fluid, or genital tissue contact. These are the routes via which chlamydia is most commonly transmitted.

It is important to note that, because the bacteria is present in mucous membranes, ejaculation does not need to take place for the infection to be transferred from a person to their partner.

  • Unprotected anal sex

Once again, this occurs when the penis comes into contact with infected tissue in the rectum; or when the latter comes into contact with infected sperm from a male.

  • Unprotected oral sex

It is possible for chlamydia to be spread via oral contact with infected sperm (male-to-female or male-to-male), or from infected tissues in the mouth to the penis (female-to-male or male-to-male). However, although possible, transmission via this route is thought to be rare. This is because the tissues in the genital area are more susceptible to infection from C. trachomatis than those in the mouth and throat.

It is therefore also possible for a female to pass chlamydia on to their partner by receiving oral sex (vagina-to-mouth), albeit not likely. Similarly, the mouth-to-vagina, mouth-to-anus and anus-to-mouth routes are all, theoretically, possible conduits for transmission, but not common.

  • Sharing sex toys

This occurs when infected semen or vaginal fluid is transferred from an infected person onto the surface of a sex toy, which is then used by their partner. In essence, the infected fluids are being transported to another person’s mucous membranes using the sex toy as a vehicle.

  • Sperm coming into contact with the eye

When infected sperm comes into contact with the exposed mucous membrane in the eye, the C. trachomatis bacteria can cause conjunctivitis. This is where the tissues surrounding the eye become reddened and inflamed, sometimes resulting in discharge.

  • Mother to baby

If an expectant mother is carrying the infection when she gives birth, then it is possible for the infection to be passed onto her baby. Chlamydia during pregnancy can also increase the likelihood of complications.

Probability of chlamydia transmission

If someone knows or suspects they have been exposed to chlamydia, they should get tested as soon as possible, so that they can have the infection diagnosed and treated if necessary. They should also abstain from sexual activities until they have been tested (or retested following treatment if applicable) and received the all-clear.

That said, if someone has had unprotected sex with an infected person, it does not necessarily mean they definitely have chlamydia.

Estimated transmission rates tend to vary and, as discussed above, the likelihood of infection depends on the sexual act performed; but Professor Victoria von Sadovszky, an expert from the Ohio State University College of Nursing, states that the transmission rate from a single unprotected exposure is thought to be around 25 percent.

Other estimates put the male-to-female transmission rate from one sexual instance at 40 percent and the female-to-male transmission rate at 32 percent.

Obviously, the more someone has sex with an infected person, the higher the chances of transmission become.

Using barrier protection can reduce the chances of picking up the infection dramatically. Provided they are used accordingly, condoms are thought to be 99 percent effective at reducing the transmission of STIs.

Find out more about preventing chlamydia transmission on our information page.

Page last reviewed:  28/02/2022