About 1 in 8 cancer cases are caused by infections with viruses, bacteria or parasites, a new study finds, suggesting that one of the most effective ways to reduce cancer rates might be to prevent common infections.

In a new analysis published Sept. 28 in the journal The Lancet Oncology, researchers mapped the global impact of 12 cancer-causing pathogens. They estimate that infections drove about 2.3 million new cancer cases in 2024, or about 12% of all cancer cases diagnosed that year.

The study highlights that a large fraction of cancer cases could be avoided if these underlying infections were prevented, Dr. Prashant Mathur, director of India's National Centre for Disease Informatics and Research, who was not involved in the study, told Live Science in an email.

"When combined with other risk factors such as tobacco use, alcohol consumption, appropriate diet, physical activity, and environmental pollution, it is estimated that about 50 to 60 percent of all cancers in India are avoidable," he said. Past studies suggest that similar figures hold on a global scale.

In the new study, the researchers used the Global Cancer Observatory's GLOBOCAN database, which combines data from various cancer registries to estimate that about 20 million new cancer cases were diagnosed in 2024. The team combined that information with other published data examining the prevalence of different infections in people with specific cancers. This enabled them to estimate how many cases were attributable to each infection.

Their analysis focused on 12 pathogens tied to cancer, including human papillomavirus (HPV), hepatitis B and C, HIV and Epstein-Barr virus (EBV), the virus behind mononucleosis. The resulting breakdown revealed that just four pathogens accounted for roughly 92% of the cancer cases caused by infections.

A bacterium called Helicobacter pylori, which infects the gastrointestinal tract, accounted for 760,000 cases, primarily gastric cancers. The other three leading cancer triggers were viruses. HPV accounted for 750,000 cases, including cervical, penile, throat and mouth cancers. The liver-attacking hepatitis B virus was responsible for 360,000 cases and EBV drove 260,000 cases. (EBV is known to increase the risk of throat cancers, as well as certain lymphomas and stomach cancers.)

The scale of the burden contrasts with gaps in prevention. In 2024, just 31% of eligible adolescent girls worldwide received at least one dose of the HPV vaccine, well below the 90% target, the World Health Organization reports. The degree of protection provided by the vaccine is striking in countries that have achieved higher coverage.

"We have now achieved the critically important milestone of cervical cancer starting to disappear in countries that quickly achieved vaccination rates of over 80 percent," said Dr. Rebecca Perkins, an obstetrician-gynecologist at Tufts University School of Medicine who was not involved in the study. "Research from Australia, England, and Finland have shown zero cases of HPV-related cancers or deaths in young adults who received vaccination by age 13," Perkins told Live Science in an email.

But these successes are concentrated in wealthy nations. The Lancet study found that low- and middle-income countries bear 77% of the global infection-driven cancer burden. For example, about 81% of HPV-linked cancers occurred in these countries.

Perkins pointed out that shifting from the standard two-dose HPV vaccine series to a single-dose regimen using the same vaccines could make mass campaigns easier worldwide by lowering costs and eliminating the need to track patients for follow-ups. The single-dose approach has been backed by recent trials.

In the U.S., progress has been uneven across states with some boasting high vaccination rates and others lagging behind. One barrier is that some children's caregivers feel the shots are unnecessary or that their kids are too young to receive it. "The biggest misconception from families is that the HPV vaccine is not needed or that it can wait," Perkins noted. "The reality is that you can't vaccinate too early, only too late."

There is also a highly effective vaccine for hepatitis B, and health officials recommend giving children their first dose as soon as possible after birth. (The U.S. Centers for Disease Control and Prevention recently made the decision to stop recommending universal vaccination at birth, which experts say could raise the country's rates of hepatitis B infections and related cancers.)

While the challenge with HPV and hepatitis B is getting their vaccines into widespread use, there isn't an approved vaccine for EBV yet.

"The biggest hurdle has been the complexity of the virus," said Claire Shannon-Lowe, a tumor virologist at the University of Birmingham in the U.K. who was not involved in the study. "Unlike many other viruses that use just one or two virus proteins to enter cells, EBV uses multiple proteins ... and it hides inside cells for our entire lifetime."

Epstein-Barr infections are also incredibly common, with up to 95% of people infected by adulthood; most people infected with the virus don't develop a related cancer, but it's still a major contributor to the global cancer burden. The germ has also been tied to several autoimmune diseases, which may help spur the development of a vaccine, Shannon-Lowe suggested.

"With the increased impetus following EBV's association with multiple sclerosis and lupus, we may finally have a potential vaccine in sight that could not only prevent virus infection but could help treat EBV-associated cancers," she said.

For H. pylori, meanwhile, researchers are pursuing population-wide testing for and treatment of the infection to help prevent gastric cancer, while vaccine development remains an area in need of further investment.

The true scale of the infection-related cancer burden may be even larger than the study suggested. Some of the registries used in the new research cover only about 19% of the global population, and only 2% of Africa's population. More high-quality data is needed to fill in these gaps.

For Mathur, though, the bottom line is that cancer prevention makes economic sense.

"Studies have clearly demonstrated that the return on investment of prevention on cancers is much higher than on its treatment in the long run," he said. "Preventive aspects must be strengthened for long-term gains."

This article is for informational purposes only and is not meant to offer medical advice.

The patient: A 61-year-old man in India

The symptoms: The man went to an emergency department after fainting. He had felt nauseous just before the episode, which lasted about three minutes and was followed by a second fainting episode lasting about two minutes. He also had swelling around one eye.

His family told doctors that he hadn't shown any jerking movements, loss of bladder control or subsequent grogginess, which would have been telltale signs of a seizure.

What happened next: Because the man had fainted twice and had a history of heart disease, doctors first looked for problems affecting his brain and heart. An MRI of his brain, used to look for structural abnormalities that could explain the fainting, and an electroencephalogram (EEG), which records the brain's electrical activity, did not reveal a cause.

The man had persistently low blood pressure, the doctors observed, and this led them to move him to the intensive care unit. There, doctors carried out tests for infections and heart attack, but all came back negative.

Meanwhile, the swelling around one of his eyes spread to both eyes and then across his face, while his blood pressure continued to fall despite treatment with intravenous fluids.

The doctors probed further into the man's medical history and activities before the fainting episodes. They learned that he had applied an artificial hair dye two days before he became ill. Soon afterward, he developed itching on his scalp, neck and face. He self-treated with the allergy medicine cetirizine, an antihistamine, for two days. He also reported spending two to three hours outdoors in the sun each day after he'd applied the hair dye.

The diagnosis: Based on the man's recent use of hair dye, the itching that followed, his rapidly worsening facial swelling and his persistently low blood pressure, doctors concluded that he was experiencing anaphylaxis, a severe allergic reaction that can affect the whole body. The reaction can be life-threatening if left untreated.

The treatment: Doctors treated the man with epinephrine (also called adrenaline), a hormonal medication that rapidly reverses the effects of anaphylaxis. They also gave him antihistamines and steroids to help bring the allergic reaction under control, along with intravenous fluids to raise his blood pressure.

However, the patient's blood pressure remained dangerously low, so doctors continued giving him epinephrine through an intravenous drip for about 16 hours. Over the next three days of treatment, his blood pressure stabilized and the swelling in his face gradually subsided.

He was transferred to a general ward and treated there for another five days before being discharged. He was advised that he could have a similar reaction in the future and referred to a dermatologist.

What makes the case unique: Most allergic reactions to hair dye remain limited to the skin, causing itching, redness and swelling where the dye has touched it. Severe reactions affecting the whole body are uncommon, the man's doctors wrote in a report of the case.

The itching the man noticed shortly after applying the dye was likely an early, mild allergic reaction, his doctors wrote. That reaction later progressed to a much more severe one, they suggested, raising the possibility of a "biphasic" allergic reaction, an uncommon pattern seen in about 5% of patients with anaphylaxis. However, this remains only a possible explanation.

The doctors also noted that the man's prolonged sun exposure after he applied the dye may have intensified the reaction; they cited previous research that ultraviolet light can enhance the allergy-triggering effects of para-phenylenediamine, a common ingredient in hair dyes. In the report, the doctors did not identify the exact hair dye ingredient that triggered the reaction in this instance.

The case highlights the importance of asking about recent use of products such as hair dye when doctors evaluate patients with otherwise unexplained fainting, facial swelling or shock, the medical team concluded.

For more intriguing medical cases, check out our Diagnostic Dilemma archives.

This article is for informational purposes only and is not meant to offer medical advice.

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