Cancer alert: 5 infections are linked to nearly 1 in 8 cancer cases worldwide. What are they?

Cancer is often associated with factors such as smoking, alcohol consumption, unhealthy diets and environmental exposure. But a significant share of the global cancer burden has another, less widely recognised contributor: infections caused by bacteria and viruses. 

An estimated 2.3 million new cancer cases worldwide in 2024 — around 12% of the total — were attributable to infections, according to a new analysis by the International Agency for Research on Cancer (IARC), the World Health Organization's specialised cancer research agency.

The findings, published in The Lancet Oncology, point to five major infectious agents responsible for the overwhelming majority of these cases. 

They are Helicobacter pylori, human papillomavirus (HPV), hepatitis B virus, Epstein-Barr virus (EBV) and hepatitis C virus. While these infections differ in how they spread and affect the body, they share a crucial characteristic: persistent infection can trigger biological changes that increase the risk of cancer. 

The findings also highlight a major public health opportunity. Vaccination, screening, early diagnosis and effective treatment can prevent or reduce the risk of several infection-related cancers. Yet access to these interventions remains uneven across countries. 

How can an infection lead to cancer? 

An infection does not automatically lead to cancer. Most people infected with these agents will not necessarily develop the disease. The risk depends on factors including the infectious agent, whether the infection persists, the body's immune response and other individual and environmental factors. 

Some infections cause long-term inflammation that damages cells and increases the likelihood of cancer-causing changes. Others interfere with the normal mechanisms that regulate cell growth or allow abnormal cells to survive and multiply. 

For instance, chronic hepatitis B and C infections can damage the liver over many years, potentially leading to cirrhosis and liver cancer. Certain high-risk types of HPV can cause persistent changes in cells that eventually develop into cancer. H. pylori, meanwhile, can produce prolonged inflammation in the stomach lining, increasing the risk of stomach cancer. 

5 infections responsible for the largest cancer burden 

1. Helicobacter pylori: The leading infectious cause of cancer: Helicobacter pylori, commonly known as H. pylori, is a bacterium that colonises the stomach lining. Infection often begins in childhood and can persist for decades if left untreated. 

The bacterium was associated with an estimated 760,000 cancer cases worldwide in 2024, making it the largest infectious contributor to cancer in the IARC analysis. That amounts to around 4% of all new cancer cases globally. 

Most people with H. pylori do not develop cancer. However, persistent infection can cause chronic gastritis and other changes that raise the risk of stomach cancer. It is also associated with a form of non-Hodgkin lymphoma affecting the stomach. 

Can it be prevented or treated? 

Yes. Doctors can diagnose H. pylori infection using appropriate tests and treat it with antibiotics, usually alongside medicines that reduce stomach acid. Research cited in the report suggests that treatment can reduce stomach cancer incidence by 36% and deaths from the disease by 22% among infected people. A vaccine against H. pylori is not yet available. 

2. Human papillomavirus: Human papillomavirus, or HPV, is an extremely common group of viruses transmitted primarily through intimate skin-to-skin contact, including sexual contact. Most infections clear naturally, but persistent infection with certain high-risk types can lead to cancer. 

The IARC analysis attributed approximately 750,000 cancer cases worldwide in 2024 to HPV, equivalent to around 4% of all new cancers. HPV is responsible for almost all cervical cancer cases. It is also linked to some cancers of the anus, penis, vagina, vulva and throat. 

Can it be prevented or treated? 

HPV vaccination is one of the most effective tools for preventing cancers associated with the virus. Studies have shown that vaccination substantially reduces persistent infections and precancerous cervical changes, while real-world evidence from countries with established vaccination programmes indicates declining cervical cancer rates. Screening is another important intervention. 

3. Hepatitis B: Hepatitis B virus (HBV) infects the liver and can cause either an acute illness or a chronic infection. When the infection persists, ongoing liver inflammation and damage can lead to cirrhosis and liver cancer. 

HBV was linked to an estimated 360,000 cancer cases worldwide in 2024, accounting for around 2% of all new cancers. The risk of chronic infection is particularly high when the virus is acquired early in life, making protection during infancy especially important. 

Can it be prevented or treated? 

Vaccination is the principal preventive measure. The World Health Organization recommends that all infants receive their first hepatitis B vaccine dose within 24 hours of birth, followed by additional doses according to the recommended schedule. A complete vaccine course provides strong, long-lasting protection against infection. Chronic hepatitis B can be managed with antiviral treatment. 

4. Epstein-Barr virus: Epstein-Barr virus (EBV) belongs to the herpesvirus family and infects an estimated 95% of people worldwide. It usually remains in the body for life after the initial infection, often without causing noticeable symptoms. 

EBV was associated with an estimated 260,000 cancer cases in 2024. It is linked to several cancers, including Burkitt lymphoma, Hodgkin lymphoma and nasopharyngeal cancer. Research has also strengthened the evidence connecting the virus with some stomach cancers. 

The widespread presence of EBV does not mean that most infected people are at risk of developing cancer. Cancer arises only in a subset of cases, through a combination of biological and other risk factors. 

Can it be prevented or treated? 

There is currently no licensed vaccine against EBV, although vaccine candidates are under development. Its widespread prevalence and links to several cancer types make the development of effective preventive tools an important research priority. 

5. Hepatitis C: Hepatitis C virus (HCV) primarily affects the liver and can cause chronic infection. Over time, persistent liver damage can lead to cirrhosis and increase the risk of liver cancer. The IARC analysis estimated that hepatitis C was responsible for around 160,000 cancer cases worldwide in 2024. 

One challenge is that hepatitis C can remain symptomless for years. People may therefore be unaware that they are infected until liver damage has already developed. 

Can it be prevented or treated? 

There is currently no approved vaccine against hepatitis C. However, direct-acting antiviral medicines can cure more than 95% of infections, often following a relatively short course of treatment. Early diagnosis and treatment are important because curing the infection substantially reduces the risk of liver cancer. 

Why the burden is higher in poorer countries 

The risk of infection-related cancer is not distributed evenly across the world. The IARC analysis found that 77% of infection-attributable cancer cases in 2024 occurred in low- and middle-income countries. 

Eastern Asia recorded the highest number of cases, with an estimated 990,000, or around 42% of the global total. The region's burden was driven largely by H. pylori and hepatitis B. 

Sub-Saharan Africa, however, had the highest proportion of cancers linked to infections: around one in four new cancer cases. HPV-related cancers contributed significantly to this burden.