Nearly all cervical cancer is caused by persistent infection with high-risk strains of human papillomavirus, commonly known as HPV. Two strains in particular, HPV 16 and HPV 18, are responsible for roughly 76% of all cervical cancer cases. But HPV infection alone isn’t enough to cause cancer. Most infections clear on their own, and it takes a combination of viral persistence, time, and additional risk factors to push normal cervical cells toward malignancy.
HPV: The Necessary First Step
HPV is an extremely common sexually transmitted infection. Most sexually active people will contract at least one strain at some point in their lives. The good news is that 80% to 90% of HPV infections are transient, meaning your immune system clears the virus within about two years without any treatment or lasting harm.
The trouble starts when a high-risk HPV infection doesn’t clear. Instead of being eliminated, the virus integrates into the DNA of cervical cells and produces proteins that interfere with your body’s normal tumor-suppressing machinery. These proteins prevent damaged cells from self-destructing the way they normally would, allowing mutations to accumulate over time. This process is slow. It typically takes 10 to 20 years for a persistent HPV infection to progress through precancerous stages and eventually become invasive cervical cancer, which is why regular screening catches most cases long before they reach that point.
Why Some HPV Infections Persist
If most HPV infections clear on their own, what determines whether yours does? The single biggest factor is immune function. Anything that weakens your immune system makes it harder to fight off the virus and gives it more time to cause cellular changes.
HIV is the most well-studied example. Women living with HIV are at least twice as likely to have cervical lesions that progress in severity compared to HIV-negative women. The virus suppresses the immune cells that would normally patrol the cervix and eliminate HPV-infected cells. This is also true for people taking immunosuppressive medications after organ transplants or for autoimmune conditions.
But you don’t need a diagnosed immune condition for persistence to occur. Chronic stress, poor nutrition, and other infections can all quietly impair your body’s ability to clear HPV, though these effects are harder to quantify than something like HIV status.
Smoking and Cervical Cell Damage
Smoking is one of the strongest non-viral risk factors for cervical cancer, and the connection is more direct than you might expect. Tobacco carcinogens don’t just stay in your lungs. Researchers have detected specific cancer-causing chemicals from cigarette smoke concentrated in cervical mucus itself. These compounds cause DNA damage in cervical cells at the same time that HPV proteins are blocking the cell’s normal repair and self-destruct mechanisms.
Smoking also appears to suppress the local immune response in the cervix, making it harder for your body to clear HPV. So it works on two fronts: weakening your defenses against the virus while simultaneously adding a second source of genetic damage to cells that have already lost their safety nets. Women who smoke have roughly double the risk of cervical cancer compared to nonsmokers.
Hormonal and Reproductive Factors
Long-term use of oral contraceptives is associated with increased cervical cancer risk, and the effect is dose-dependent. Using birth control pills for fewer than five years carries about a 10% increased risk. Five to nine years of use raises the risk by about 60%, and ten or more years of use roughly doubles it. Importantly, this elevated risk appears to decline after stopping the pill, gradually returning toward baseline over about ten years.
The biological explanation isn’t fully settled. Hormones in oral contraceptives may make cervical cells more susceptible to HPV infection, or they may influence how the virus behaves once it’s established. Having many full-term pregnancies is also associated with higher risk, possibly because hormonal changes during pregnancy affect the cervix in similar ways, or because pregnancy temporarily suppresses certain immune responses.
Other Infections That Raise Risk
Chlamydia, one of the most common sexually transmitted bacterial infections, appears to act as a significant cofactor alongside HPV. Chlamydia damages the lining of the cervix, creating small breaks in the tissue that make it easier for HPV to reach the deeper cell layers where it establishes infection. Chlamydial infection also disrupts the local immune response needed to clear the virus.
The combination is particularly concerning. Women coinfected with both HPV and chlamydia have roughly four times the risk of cervical cancer compared to women without either infection. Repeated chlamydia infections increase the risk of precancerous cervical changes even further, especially among women who already have a persistent high-risk HPV infection. Because chlamydia often causes no symptoms, many women don’t know they have it, which is one reason routine STI screening matters for cervical cancer prevention as well.
Rare Non-HPV Causes
A very small percentage of cervical cancers develop without HPV involvement. The best-documented example involves diethylstilbestrol (DES), a synthetic estrogen prescribed to pregnant women between 1940 and 1971 to prevent miscarriage. Daughters of women who took DES during pregnancy face about 40 times the normal risk of developing a specific type called clear cell adenocarcinoma of the cervix and vagina. Even so, this cancer remains rare, affecting roughly 1 in 1,000 DES-exposed daughters. The risk persists as these women age into their 40s and 50s. Since DES was pulled from the market over 50 years ago, this cause is becoming increasingly uncommon, but affected women are still advised to have annual examinations.
How Screening Catches Cancer Early
Because cervical cancer develops over many years through identifiable precancerous stages, screening is remarkably effective at catching it before it becomes dangerous. Two main tests are used: the Pap smear, which examines cervical cells under a microscope, and the HPV DNA test, which checks directly for the presence of high-risk HPV strains.
HPV testing is considerably more sensitive. In large studies, HPV DNA testing detected about 96% of significant precancerous lesions, compared to roughly 53% for the Pap smear alone. The tradeoff is that HPV testing flags more infections that would have cleared on their own, leading to slightly more follow-up procedures. Many current screening guidelines use both tests together or use HPV testing as the primary screen, with Pap smears reserved for triage when HPV is detected.
How Vaccination Changes the Picture
The HPV vaccine has dramatically altered cervical cancer risk for vaccinated populations. Among women vaccinated between ages 12 and 13, researchers have documented an 87% reduction in cervical cancer rates and a 97% reduction in the most severe precancerous lesions. In Denmark, vaccination before age 17 was associated with an 86% reduction in cervical cancer incidence.
Vaccination is most effective when given before any exposure to HPV, which is why it’s recommended for preteens. But it still provides meaningful protection when given later, up to age 26 for most people and up to 45 in some cases. The vaccine covers the highest-risk HPV strains, including types 16 and 18, but not every cancer-causing strain. This is why screening remains important even for vaccinated individuals, though the overall risk drops substantially.