Nearly all cervical cancer is caused by a persistent infection with human papillomavirus, commonly known as HPV. Two specific strains, HPV 16 and HPV 18, are responsible for roughly 76% of all cervical cancers worldwide. The infection itself is extremely common and usually clears on its own, but when it doesn’t, it can trigger a slow chain of cellular changes that, over 15 to 20 years, may develop into invasive cancer.
HPV: The Primary Cause
HPV is a group of more than 200 related viruses, about 14 of which are classified as “high-risk” for cancer. Among those, types 16 and 18 are by far the most dangerous. Most sexually active people will contract some form of HPV at some point in their lives, and in most cases the immune system eliminates the virus within a year or two without any symptoms or lasting harm.
The problem begins when a high-risk strain persists. The virus produces two proteins that essentially disable the cell’s built-in safety mechanisms. One protein breaks down p53, a molecule that normally stops damaged cells from multiplying. The other interferes with a second safeguard called Rb, which controls the timing of cell division. With both brakes removed, infected cervical cells can accumulate genetic damage and divide unchecked. This doesn’t happen overnight. Precancerous changes typically progress through stages over many years before becoming invasive, which is why regular screening can catch the process early.
Why Only Some HPV Infections Lead to Cancer
Given how common HPV is, the fact that cervical cancer develops in a relatively small number of women tells you that the virus alone isn’t enough. Several factors influence whether an HPV infection clears harmlessly or persists long enough to cause trouble.
Immune Health
Your immune system is the main line of defense against persistent HPV. Specialized immune cells called CD4+ T cells are responsible for recognizing and clearing the virus. Anything that weakens this response gives HPV more time to integrate into cervical cell DNA and begin driving abnormal growth. HIV is the most significant example. HIV directly depletes CD4+ T cells, and research shows that women with both HPV and HIV develop more severe cervical lesions that progress faster than in women with HPV alone. Other conditions that suppress the immune system, such as organ transplant medications, carry a similar increased risk.
Smoking
Cigarette smoking delivers carcinogens directly to cervical tissue. Research published in the Journal of the National Cancer Institute found measurable levels of a potent tobacco-specific carcinogen called NNK in the cervical mucus of smokers. Nicotine and its metabolites travel through the bloodstream and concentrate in cervical fluid, where enzymes already present in cervical cells can convert these compounds into forms that react with and damage DNA. This smoking-related DNA damage has been confirmed across multiple studies. In practical terms, smoking both raises the likelihood that an HPV infection will persist and independently damages cervical cells, creating a compounding effect.
Long-Term Oral Contraceptive Use
Extended use of oral contraceptives is associated with a higher risk of cervical cancer, though the reasons aren’t entirely clear. Women who used the pill for five to nine years were nearly three times more likely to develop cervical cancer than non-users. For those who used it for more than ten years, the risk was about four times higher. The risk appears to decline after stopping. This doesn’t mean the pill causes cancer on its own, but in women who already have a persistent HPV infection, hormonal changes may create conditions that favor the virus’s progression.
Other Sexually Transmitted Infections
Chlamydia, one of the most common bacterial sexually transmitted infections, appears to raise cervical cancer risk through several pathways. The infection causes chronic inflammation in cervical tissue, generating free radicals that damage DNA and impair the cell’s ability to repair itself. Chlamydia also disrupts the connections between cervical cells, potentially making it easier for HPV to reach the deeper layers of tissue where it establishes infection. Studies with up to 19 years of follow-up have found that repeated chlamydia infections increase the risk of high-grade precancerous changes, particularly in women who already carry high-risk HPV.
How Cervical Cancer Develops Over Time
The journey from initial HPV infection to invasive cervical cancer is one of the slowest progressions of any cancer, typically spanning 15 to 20 years. This long timeline is both the challenge and the opportunity with cervical cancer.
After persistent HPV infection takes hold, cells in the cervix begin to show abnormal changes classified in stages. Early changes are mild and often reverse on their own. More advanced precancerous changes are less likely to resolve without treatment but are still highly treatable if caught. The critical point is that precancerous stages rarely cause symptoms. There’s no pain, no bleeding, no obvious sign that cells are changing. This is why screening exists: to catch the process during the long window before it becomes cancer.
Screening That Catches It Early
Because the cause of cervical cancer is well understood and the timeline is long, screening is remarkably effective at prevention. Current guidelines from the American Cancer Society recommend HPV testing every five years when a clinician collects the sample, or every three years when a self-collected vaginal sample is used. A negative HPV test is reassuring because it means the primary cause of cervical cancer isn’t present.
The WHO has set a global target of screening 70% of women by age 35 and again by age 45, as part of a broader initiative aiming to eliminate cervical cancer as a public health problem. The other two pillars of that effort are vaccinating 90% of girls by age 15 and treating 90% of women identified with precancerous or cancerous changes.
HPV Vaccination and Prevention
Because HPV causes the vast majority of cervical cancers, vaccination against the virus is the most direct form of prevention available. A large Swedish study of nearly 1.7 million women found that girls vaccinated before age 17 had a nearly 90% reduction in cervical cancer incidence over an 11-year period compared to unvaccinated women. The benefit was strongest in those vaccinated before any exposure to the virus, which is why vaccination is recommended in early adolescence.
Eight HPV vaccines are now licensed globally, and all protect against HPV types 16 and 18. The current vaccine used in the United States covers nine HPV types, offering broader protection against additional cancer-causing strains as well as types that cause genital warts. Vaccination doesn’t treat existing infections, but for those who haven’t yet been exposed, it effectively removes the root cause of most cervical cancers.