In the microscopic world of germs, organisms called Chlamydiae are dwarfed by their fellow bacteria. An E. coli bacterium can hang out with 100,000 of its closest friends on the head of a pin, but Chlamydiae are smaller still. Infectious particles are about one-tenth the length of an E. coli, rivaling the size of a large virus. And, just like a virus, Chlamydiae can still pack quite a punch, proving that sometimes, not-so-good things can come in small packages.

There are many types of Chlamydiae bacteria, but one species, Chlamydia trachomatis, is responsible for not one, but two sexually transmitted diseases (STDs) in humans: chlamydia and lymphogranuloma venereum (LGV). (Humans aren’t the only ones affected by sexually transmitted Chlamydiae. A different species, Chlamydia pecorum, is devastating wild koalas in Australia, which has got to be one of the biggest bummers ever.)

Chlamydia is a case study for the importance of safer sex and regular STD testing.

Chlamydia is one of the most common STDs in the United States — there were almost 1.5 million diagnoses in 2011 alone, but experts estimate that there were around another 1.5 million cases of chlamydia that went undiagnosed. How can this be? Chlamydia is often a “silent” infection, meaning that symptoms are rare, allowing people to harbor these bacteria without even knowing it. (When symptoms do occur, they might include swelling in the genital region; vaginal, cervical, or penile discharge; or painful urination.)

It might seem like a small mercy that this common infection is unlikely to torture us with harrowing symptoms — but, in actuality, those of us who have to deal with discharge or burning urination should try to appreciate the heads up: Left untreated, chlamydia can cause serious complications. When it spreads along the female reproductive tract, it can cause pelvic inflammatory disease, which can severely compromise fertility and cause chronic pain. Rarely, in a male reproductive tract, it can cause epididymitis, which can also spell bad news for future fertility.

LGV is uncommon in the United States — there are only around 200 to 400 documented cases each year, most commonly affecting men who have sex with men. But LGV usually manifests itself in tropical climes. This particular infection is caused by strains of C. trachomatis that can invade more tissue types than its relatively tamer temperate counterparts. If untreated, it can do serious tissue damage, potentially culminating in genital ulcers, fistulas, rectal strictures, and genital elephantiasis.

A C. trachomatis bacterium is a ball-shaped bug that can sneak into our cells. Although our bodies quickly dispatch defense forces, which quarantine the invader in an enclosure called a vacuole, the bacterium just uses that time alone to reproduce, which it does asexually. Eventually, a huge collection of clones bursts from the host cell, killing it while loosing new infectious particles into the body, starting the cycle anew.

These stripped-down organisms are so minimalistic that they can’t take care of themselves outside of our cells — unlike most organisms, they can’t make their own energy, vitamins, amino acids, or other necessary compounds. In order to survive, they need to parasitize our cells. Infectious bacteria produce a toxin, and, ironically, if too many of them invade a host cell at once, this toxin will kill it before the bacteria even have a chance to reproduce.

The absolute best way for sexually active people to protect themselves from this wily bacterium is to use condoms consistently and correctly. Unless you and your monogamous partner have both tested negative for chlamydia at the outset of your relationship, put a condom on the penis (or on shared penetrative sex toys) before engaging in vaginal, anal, or oral sex. And, because chlamydia can be transmitted without ejaculation, make sure that a condom is in use from start to finish.

No penises at play? Dental dams can also reduce risk, and can be used for protection during oral contact with the vagina or anus. Chlamydia infections have been reported in lesbian populations, although the transmission rate among women who have sex with women hasn’t been studied too thoroughly.

If you have symptoms of chlamydia, you should definitely be tested, which can be easily accomplished by peeing into a cup. No symptoms? The Centers for Disease Control and Prevention recommends chlamydia testing in anyone whose partner has been recently diagnosed with an STD; in pregnant people; in sexually active females 25 or younger; in females with new sexual partners or multiple partners; and in males who have receptive anal sex with other males. Still undecided? Check out The Check, a widget that can help you determine if you should be tested for chlamydia and other STDs.

Affordable and nonjudgmental chlamydia testing can be obtained at any Planned Parenthood health center, and if you test positive it can be cured with antibiotics. Make sure your partner or partners receive treatment too, otherwise you risk reinfection, which would also be a big bummer. To further protect yourself, grab a handful of condoms, which are plentiful at our health centers.

Click here to check out other installments of our monthly STD Awareness series!