Pelvic inflammatory disease (PID) is an infection of the upper reproductive tract, including the uterus, fallopian tubes, and ovaries, caused by bacteria that travel upward from the vagina or cervix. An estimated 2.5 million women of reproductive age in the United States have been diagnosed with it, and the actual number is likely higher because many cases produce mild or no symptoms. While sexually transmitted infections are the most well-known trigger, PID can also develop from bacteria that normally live in the vagina.
How Bacteria Reach the Upper Reproductive Tract
PID starts in the lower genital tract. Bacteria that are present in the vagina or cervix migrate upward through the cervical opening and into the uterus, then potentially into the fallopian tubes and surrounding pelvic tissue. This ascending path is the core mechanism behind virtually all cases of PID.
The cervix normally acts as a partial barrier, producing mucus that limits bacterial movement. But certain conditions weaken that barrier. An active STI can inflame the cervix and make it easier for bacteria to pass through. Hormonal changes during menstruation thin the cervical mucus, which is one reason symptoms sometimes begin during or just after a period. Any procedure that opens the cervix, such as an IUD insertion or an endometrial biopsy, can also briefly create a pathway for bacteria, though this risk is small.
Sexually Transmitted Bacteria
Chlamydia and gonorrhea are the two infections most strongly linked to PID. Chlamydia is the more common culprit, partly because it frequently causes no symptoms on its own, allowing it to persist untreated for weeks or months. During that time, the bacteria can slowly ascend into the upper tract. Gonorrhea tends to cause more obvious symptoms but can also be silent, especially in women.
A newer concern is Mycoplasma genitalium, a sexually transmitted bacterium that the CDC notes may play a role in PID. It tends to produce milder symptoms than chlamydia or gonorrhea, which makes it harder to detect. Testing for it is not yet routine in most clinics, so it can go undiagnosed even when a provider is actively looking for STI-related causes of pelvic pain.
Non-STI Bacteria
Not every case of PID traces back to a sexually transmitted infection. A significant portion of cases involve bacteria that are part of the normal vaginal environment. These include anaerobic bacteria (organisms that thrive without oxygen), Gardnerella vaginalis (the bacterium most associated with bacterial vaginosis), certain strep species, and enteric gram-negative bacteria, which are the type normally found in the intestinal tract.
When the balance of vaginal bacteria shifts, these organisms can overgrow and become capable of causing infection in the upper tract. Bacterial vaginosis, a condition defined by exactly this kind of imbalance, is a recognized risk factor for PID. In BV, the protective lactobacilli that normally dominate the vagina are replaced by a mix of anaerobic bacteria. That altered environment makes it easier for harmful organisms to gain a foothold and travel upward.
In some cases, multiple types of bacteria are involved simultaneously. PID is often polymicrobial, meaning a mix of STI-related and vaginal bacteria work together to cause the infection. This is one reason treatment typically involves antibiotics that cover a broad range of organisms rather than targeting a single bacterium.
Risk Factors That Increase Vulnerability
Age is one of the strongest risk factors. Women under 25 are disproportionately affected, in part because the cervix in younger women is more susceptible to infection and because STI rates are highest in this age group.
Having multiple sexual partners or a new sexual partner raises risk, as does having a partner who has other sexual partners, even if you don’t. Unprotected sex is the most direct risk factor because barrier methods like condoms reduce exposure to STI-causing bacteria.
Vaginal douching disrupts the protective bacterial balance in a way that specifically favors PID. Douching reduces the population of hydrogen-peroxide-producing lactobacilli, the beneficial bacteria that help keep the vaginal environment acidic and hostile to pathogens. The resulting shift toward mixed anaerobic bacteria creates conditions similar to bacterial vaginosis, which in turn raises the risk of ascending infection. A previous episode of PID is also a significant risk factor, as the tissue damage from the first infection can make the reproductive tract more vulnerable to reinfection.
Why Many Cases Go Unnoticed
PID does not always announce itself with obvious symptoms. Some women experience sharp pelvic pain, abnormal discharge, painful urination, or bleeding between periods. But many others have symptoms so mild they attribute them to menstrual cramps or a minor stomach issue. Some have no symptoms at all.
This subclinical form of PID is particularly concerning because the infection can still cause damage to the fallopian tubes even without producing pain. Scar tissue forms silently, and the first sign of a problem may come years later when a woman has difficulty getting pregnant or develops an ectopic pregnancy. The CDC’s national survey data showing 4.4% of sexually experienced reproductive-age women reporting a lifetime PID diagnosis almost certainly underestimates the true burden, since it only captures cases that were actually identified and diagnosed.
How PID Leads to Long-Term Damage
The fallopian tubes are the most vulnerable part of the reproductive tract during a PID episode. Infection causes inflammation and swelling inside the tubes, and as the body heals, scar tissue can partially or completely block them. This scarring is the link between PID and both infertility and ectopic pregnancy.
The damage is cumulative. After a single episode of PID, roughly 13% of women develop tubal blockage. After two episodes, that number jumps to about 36%. After three or more infections, the rate reaches 75%. Each episode compounds the scarring, which is why prompt treatment of any suspected case matters so much.
Ectopic pregnancy, where a fertilized egg implants in a fallopian tube instead of the uterus, occurs when scarring partially blocks the tube. The egg can still be fertilized but cannot pass through to the uterus. This is a medical emergency. Chronic pelvic pain is another common consequence, caused by adhesions (bands of scar tissue) that form between pelvic organs during the healing process.
What Triggers a First Episode
For most women, PID develops through a specific chain of events. An STI or a shift in vaginal bacteria goes untreated, bacteria migrate upward through the cervix, and infection takes hold in the upper reproductive tract. The timeline varies. Some women develop PID within days of an initial infection, while others carry the triggering bacteria for months before the ascending infection occurs.
There is no single moment that “causes” PID in most cases. It is the combination of bacterial exposure, a cervix that allows passage, and enough time without treatment for the infection to establish itself in the upper tract. This is why regular STI screening, especially for chlamydia in women under 25, is one of the most effective ways to prevent PID. Catching and treating the lower-tract infection before bacteria have a chance to ascend eliminates the most common trigger in the chain.