Causes and Risk Factors of Chlamydia

Chlamydial diseases are sexually transmitted and caused by the bacterium Chlamydia trachomatis. However, this bacterium acts more like a virus. This can affect the way chlamydia infection is transmitted and the risk factors that are important in acquiring it. Chlamydia infections can affect the vagina, cervix, and rectum, among other areas.

Fortunately, chlamydia is a largely preventable infection. Learning how Chlamydia trachomatis behaves can give you a better understanding of what makes an infection more likely. 

chlamydia risk factors
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Chlamydia Bacterium

Most bacteria are capable of reproducing on their own as long as they're in a hospitable environment—but not the type associated with chlamydia. The chlamydia bacterium must rely on its hosts (humans) to survive, much like a virus does.

Essentially, chlamydia treats the insides of human cells like great big grocery stores. From the host, it takes ATP (an energy molecule), nutrients, and other elements that are essential for reproduction and that the bacterium can't make on its own.

Since the bacterium can't live without these necessities, Chlamydia trachomatis is an obligate (it can't survive without) intracellular (living inside cells) parasite (where it takes but does not give back).


Chlamydia basically has a two-phase life cycle: the elementary body and reticulate body stages.

Elementary Body

Chlamydia travels between cells, and between people, in the form of an elementary body—a small, dense, spore-like structure.

In this stage, chlamydia doesn't do much of anything. Bacteria travel between cells and between people to create new infections, but these bodies don't replicate or change. They are just carried around in bodily fluids.

Chlamydia is infectious, but not active in this stage.

Reticulate Body

Chlamydia enters this stage once the elementary body infects a new cell. In this form, the bacterium use supplies from the host cell to make copies of itself inside the cell.

Reticulate bodies can grow, divide, and metabolize. Infections can persist in this manner for a while.

Once there are enough copies—too many to survive inside the cell—reticulate bodies can turn back into elementary bodies, burst the host cell open, and escape to infect new cells (either in the infected individual or a sexual partner). This starts the process all over again.

This is a pretty strange life cycle that doesn't really follow the roadmap for either a bacterial or viral infection. That is one of the reasons that chlamydia is so interesting and important to study.


Discussing the characteristics of chlamydia is important because its characteristics affect the way the bacterium is transmitted from person to person. The method of transmission, in turn, affects the risk factors that make it more likely a person will contract the infection.

Chlamydia is transmitted through secretions rather than skin-to-skin contact, as is the case with some microorganisms (such as human papillomavirus, or HPV). This means it is less likely to pass between two people without some form of bodily fluid, such as semen or cervical mucus, present. It also means that condoms can be very effective at preventing the spread of this type of bacteria.

Understanding the elementary body stage also helps clarify why sometimes chlamydia infections are present for months or even years before they are detected. This is especially important if you have a partner who, upon learning of your chlamydia diagnosis, wonders how you became infected despite being in a committed sexual relationship with them for a lengthy period of time.

The risk factors for chlamydia are similar to the risk factors for sexually transmitted infections (STIs) in general, but can vary somewhat based on the method of transmission.

Lifestyle Risk Factors

Certain lifestyle practices can increase your risk of chlamydia infection, including:

  • Unprotected sex: Engaging in either vaginal, receptive anal, or oral sex without a condom is the greatest risk factor for developing chlamydia. Since bacteria are spread by secretions, using a condom every time you have sex (unless you're in a long-term monogamous relationship in which both partners have tested negative) is the best way to avoid an infection.
  • Having multiple sex partners: The more sex partners a person has, the more likely it is that they will develop an STI, including chlamydia. Of course, it only takes one sex partner to transmit the infection, and practicing safe sex is important no matter your sexual practices.
  • A partner who has an STI: Obviously, an untreated partner poses a risk. But there is also a risk of transmission if a partner has not yet finished a full seven-day course of antibiotics, or if they received a single-dose medication and seven days have not yet passed.
  • Men who have sex with men (MSM): Men who have sex with men are more likely to develop genital, rectal, and/or oral chlamydia infection than heterosexual men. In a study, 11.8% of MSM in an urban area of the United States were found to have extragenital chlamydia infections involving either the anus or the throat.

Health Risk Factors

People with certain existing health concerns are at greater risk for chlamydia infection than others.

Health risk factors include:

  • Having other STIs: Lifestyle practices that can predispose you to another STI can also raise your risk of chlamydia (and vice versa). Many STIs also cause inflammation of the sensitive mucosa of the vagina, cervix, or urethra. When this tissue is compromised due to one infection, it's easier for another microorganism to enter the body and begin to grow.
  • Being HIV positive: Up to 10% of men who are positive for human immunodeficiency virus (HIV) will also test positive for chlamydia.
  • Having cervical ectopy: Cervical ectopy, a condition in which the cells of the endocervix (cervical canal) are instead found on the ectocervix (outside the cervical canal), increases the susceptibility of the tissue to chlamydial infection. This condition is most common in young women. An older study looking at women aged 15–24 found that those with cervical ectopy were almost twice as likely to test positive for chlamydia. As women age, the cervical tissue migrates and cervical ectopy usually goes away, putting them at lower risk for chlamydia.

Untreated mothers can also pass chlamydia to their babies during birth.


Unlike some infections, in which a person develops immunity after exposure, the body does not develop any immunity against chlamydia after an infection. This means that you can be infected over and over again. 


Reducing your risk of contracting chlamydia and practicing safe sex begins with choosing your sex partners wisely.

While asking a potential partner about previous diagnoses may not exactly be something you'd like to do, know that people are having these important conversations much more often now than in the past. Protecting your health is nothing to feel embarrassed about.

The most effective way to prevent chlamydia, specifically, is to use a condom each and every time you have vaginal or anal sex.

Reducing your risk with oral sex is also possible. Condoms can be used during fellatio, and dental dams or other barriers can be used during rimming or cunnilingus.

Even if you are careful, it's still important to see your healthcare provider regularly and undergo routine screening for chlamydia. Only 5%–30% of infections in women and only 10% of infections in men cause symptoms. Being tested is the only way to know if you're infected for sure—and to prevent the complications of an untreated case.

The Doctor Discussion Guide below can help you start that conversation with a healthcare professional.

Chlamydia Doctor Discussion Guide

Get our printable guide for your next doctor's appointment to help you ask the right questions.

Doctor Discussion Guide Woman

Frequently Asked Questions

  • What causes chlamydia?

    Chlamydia is a sexually transmitted infection (STI) caused by the bacterium Chlamydia trachomatis. This specific type of bacteria is unique in that it behaves like a virus by relying on a human host to survive and replicate. This also affects the way the infection is transmitted.

  • How do you get chlamydia?

    Chlamydia is transmitted through unprotected vaginal, anal, or oral sex. It is passed via bodily secretions, such as semen or cervical mucus (but not saliva), meaning it is unlikely to pass from person to person through skin-to-skin contact alone. Certain lifestyle factors can increase your risk:

    • Engaging in unprotected sex
    • Having multiple sex partners
    • Having another STI or having a partner who has an STI
    • Men who have sex with men (MSM)
    • Being HIV positive
    • Having cervical ectopy
  • How long can chlamydia go untreated before it causes damage?

    There's no specific timeline, but the longer chlamydia goes untreated, the more damage can occur from complications such as pelvic inflammatory disease. Known as a silent disease, chlamydia presents with symptoms in only about 5%–30% of cases in women and only about 10% of cases in men, which means the infection may often go untreated for a while before it's discovered. When symptoms do appear, they usually don't show up until three weeks after the initial exposure.

  • Can you get chlamydia without being sexually active?

    No. Chlamydia is passed only through sexual contact with an infected person, but ejaculation does not have to occur for transmission. However, chlamydia may also be spread from an untreated mother to her newborn during birth.

  • Can you get chlamydia more than once?

    Yes. Unlike some other infections, the body does not form immunity to chlamydia, which means infections can keep occurring. Fortunately, it can be treated with antibiotics.

4 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Elwell C, Mirrashidi K, Engel J. Chlamydia cell biology and pathogenesis. Nat Rev Microbiol. 2016;14(6):385-400. doi:10.1038/nrmicro.2016.30

  2. Centers for Disease Control and Prevention. Chlamydia—CDC fact sheet (detailed).

  3. Trebach JD, Chaulk CP, Page KR, Tuddenham S, Ghanem KG. Neisseria gonorrhoeae and Chlamydia trachomatis among women reporting extragenital exposures. Sex Transm Dis. 2015;42(5):233-9. doi:10.1097/OLQ.0000000000000248

  4. Jones J, Weiss K, Mermin J, et al. Proportion of incident human immunodeficiency virus cases among men who have sex with men attributable to gonorrhea and chlamydia: A modeling analysis. Sex Transm Dis. 2019;46(6):357-363. doi:10.1097/OLQ.0000000000000980

Additional Reading

By Elizabeth Boskey, PhD
Elizabeth Boskey, PhD, MPH, CHES, is a social worker, adjunct lecturer, and expert writer in the field of sexually transmitted diseases.