What Infection Causes Bleeding Between Periods?

Bleeding between periods has many possible causes, and infection is one of them. The infections most often linked to intermenstrual bleeding are sexually transmitted infections such as chlamydia and gonorrhea, and pelvic inflammatory disease, which is a complication of those infections. Cervicitis — inflammation of the cervix — is the specific condition that most directly produces spotting or bleeding between periods.

That said, infection is not the most common cause. Hormonal changes, fibroids, polyps, and other non-infectious conditions cause bleeding between periods more often than infections do. Still, infection matters because it is treatable and because untreated STIs can lead to serious complications.

What Infection Causes Bleeding Between Periods?

Chlamydia and gonorrhea are the two infections most commonly associated with bleeding between periods. Both can cause cervicitis, which is inflammation of the cervix. When the cervix is inflamed, its surface tissue becomes fragile and bleeds easily — sometimes after sex, sometimes on its own.

Pelvic inflammatory disease, often called PID, is another major cause. PID usually develops when chlamydia or gonorrhea goes untreated and spreads from the cervix into the uterus, fallopian tubes, and ovaries. Bleeding between periods is one symptom of PID.

Other infections that can cause spotting include:

• Trichomoniasis — a parasitic infection that can inflame the cervix and vagina
• Mycoplasma genitalium — an emerging STI that some research links to cervicitis and irregular bleeding
• Bacterial vaginosis — a disruption of normal vaginal bacteria; it more often causes discharge and odor than bleeding, but it can occasionally contribute
• Endometritis — inflammation of the uterine lining, which can result from an STI or from a procedure such as childbirth or abortion

It is worth noting that many people with chlamydia or gonorrhea have no symptoms at all. When symptoms do appear, they may include unusual discharge, burning during urination, or bleeding between periods. The absence of symptoms does not mean the infection is gone.

How Do These Infections Cause Bleeding?

The mechanism is fairly straightforward. The cervix is covered by a thin layer of cells. Infection triggers an immune response that brings blood flow and inflammatory chemicals to the area. That inflammation makes the tissue swollen and fragile.

When the surface of the cervix is inflamed, even minor contact — intercourse, a pelvic exam, or normal movement — can cause small blood vessels to break. That blood shows up as spotting or light bleeding between periods.

In PID, the infection moves higher into the reproductive tract. The lining of the uterus becomes inflamed, and that inflammation can cause irregular bleeding or bleeding after sex. If the infection reaches the fallopian tubes, it can cause scarring. That scarring is what leads to long-term complications like infertility and ectopic pregnancy.

One thing that is often misunderstood: bleeding between periods is not a sign that the infection is “mild” or “just starting.” Some people with significant infection have no bleeding at all, and some with light bleeding have a serious infection. The amount of bleeding does not reliably indicate how severe the infection is.

What Other Symptoms Point to an Infection vs. Something Else?

Infection-related bleeding often comes with other signs. These can help distinguish it from hormonal or structural causes.

Symptoms that suggest infection include:

• Unusual vaginal discharge, especially yellow or green, or with a strong odor
• Pain or burning during urination
• Pain during sex
• Pelvic or lower abdominal pain
• Fever (more common with PID)
• Bleeding after sex

Symptoms that suggest a non-infectious cause include:

• Bleeding that follows a predictable hormonal pattern, such as mid-cycle spotting
• Heavy or prolonged periods along with bleeding between them, which can point to fibroids or polyps
• Bleeding associated with starting, stopping, or missing birth control pills
• Spotting in the first months after starting a new hormonal contraceptive

These patterns are not perfect. Some people with infection have only bleeding and nothing else. Some people with fibroids have discharge. The only way to know for certain is testing.

How Are These Infections Diagnosed?

Diagnosis starts with a medical history and a pelvic exam. During the exam, a clinician looks at the cervix and may see signs of inflammation, such as redness, easy bleeding, or discharge.

Testing for chlamydia and gonorrhea is typically done with a urine sample or a swab of the cervix or vagina. These tests are highly accurate. Results usually come back within a few days.

Trichomoniasis is often diagnosed with a vaginal swab examined under a microscope or with a nucleic acid amplification test, which detects the organism’s genetic material.

PID is usually diagnosed based on symptoms and exam findings rather than a single test. There is no simple blood test that confirms PID. Because untreated PID can cause permanent damage, some clinicians start treatment based on clinical suspicion rather than waiting for test results. This is a case where common practice and evidence-based guidance align — early treatment matters.

If initial tests are negative and bleeding continues, further evaluation may include ultrasound, endometrial biopsy, or hysteroscopy to look for non-infectious causes.

How Are Infection-Related Causes Treated?

Bacterial STIs — chlamydia, gonorrhea, trichomoniasis, and PID — are treated with antibiotics. The specific antibiotic and duration depend on the infection and local resistance patterns. Treatment guidelines are updated periodically by public health authorities, so the exact regimen your clinician prescribes may differ from what was standard a few years ago.

Viral infections like herpes can also cause cervical inflammation and bleeding, but they are managed differently. Herpes is not cured by antibiotics. Antiviral medications can reduce outbreaks and lower the risk of transmission, but they do not eliminate the virus.

For any STI, treatment of sexual partners is important. Reinfection is common when partners are not treated. Many public health programs offer expedited partner therapy, where medication is provided to partners without requiring them to be seen first.

Follow-up testing is often recommended to confirm the infection has cleared, especially for chlamydia and gonorrhea. This is not because treatment usually fails, but because reinfection is common and because some strains of gonorrhea have developed antibiotic resistance.

When Should You See a Doctor?

Any bleeding between periods deserves medical attention. Most causes are not dangerous, but some — including certain infections and, rarely, precancerous or cancerous changes — need timely treatment.

See a doctor promptly if you have:

• Bleeding between periods that lasts more than one cycle
• Bleeding after sex
• Pelvic pain, especially with fever
• Unusual discharge with a strong odor
• Bleeding that is heavy enough to require a pad or tampon

If you are pregnant, any bleeding should be evaluated right away. The causes and risks are different during pregnancy, and the guidance for pregnant patients differs from the guidance for non-pregnant patients.

If you have had a new sexual partner or multiple partners, regular STI screening is recommended even without symptoms. Many infections are silent.

What Else Can Cause Bleeding Between Periods?

Infection is only one category. Other common causes include:

• Hormonal imbalances — thyroid problems, perimenopause, polycystic ovary syndrome
• Fibroids — non-cancerous growths in the uterus
• Polyps — small growths on the cervix or uterine lining
• Birth control — especially during the first few months or when pills are missed
• Endometriosis and adenomyosis — conditions where uterine-like tissue grows outside its usual location
• Precancerous or cancerous changes — in the cervix, uterus, or vagina
• Trauma or foreign objects — including a retained tampon or an IUD

Age matters. In people in their 20s and 30s, infection and hormonal causes are more common. After menopause, any bleeding is considered abnormal until proven otherwise and should always be evaluated.

The key point is that bleeding between periods is a symptom, not a diagnosis. It tells you something is happening, but not what. Testing is the only way to know.