Viruses are tiny, infectious microbes. They’re technically parasites because they require a host cell to reproduce. Upon entry, the virus uses components of the host cell to complete its life cycle.

Some viruses can cause or contribute to the development of cancer. These viruses are called oncogenic viruses.

Unlike other viruses, such as influenza viruses, that cause an acute infection, oncogenic viruses often cause long-term, persistent infections.

It’s estimated that viruses account for about 20 percent of cancers. And there may be more oncogenic viruses that experts aren’t aware of yet.

EBV is a type of herpes virus. You may be familiar with it as the cause of infectious mononucleosis, or mono.

EBV is most often spread through saliva. It can be contracted through coughing, sneezing, and close contact, such as kissing or sharing personal items.

The virus can also be spread through blood and semen. This means you can encounter it through sexual contact, blood transfusions, or organ transplants.

Most EBV infections occur during childhood, although not everyone who contracts the virus has symptoms. Once you’ve contracted it, it remains in your body for the rest of your life. But it eventually lies dormant in your body.

Mutations that occur in cells due to EBV infection may contribute to certain rare cancers, including:

HBV causes viral hepatitis. Hepatitis is an inflammation of the liver. Many people with HBV go on to recover following an acute infection. However, some develop a chronic (long-term) HBV infection.

The virus spreads through bodily fluids, including blood, semen, and vaginal secretions.

Common ways infection can occur include:

  • having unprotected sexual activity with someone who has the virus
  • sharing needles
  • sharing personal items that could contain blood, including razors and toothbrushes
  • transmitting the virus to an infant during birth, if the mother has HBV

Having a chronic HBV infection leads to liver inflammation and damage, which are risk factors for liver cancer.

Like HBV, HCV also causes viral hepatitis.

According to the American Cancer Society, HCV is less likely than HBV to cause symptoms. But it’s more likely to cause a chronic infection. As a result, some people may have an HCV infection and not know it.

HCV spreads the same way HBV does. However, sexual activity seems to be a slightly less common cause of HCV transmission.

Similarly to HBV, a chronic HCV infection can lead to prolonged liver inflammation and damage, increasing a person’s risk of liver cancer.

HIV is a retrovirus that can lead to the development of AIDS.

HIV infects and destroys cells in the immune system called helper T cells. As the numbers of these cells decline, the immune system has a harder time fighting infections.

HIV spreads through bodily fluids, including blood, semen, and vaginal fluids.

Some ways that transmission can occur include:

  • unprotected sexual activity with someone who has the virus
  • sharing needles
  • sharing personal items that could contain blood, including razors and toothbrushes
  • transmitting the virus to an infant during birth, if the mother has HIV

It’s important to note that HIV doesn’t cause cancer by itself. The immune system is important in both fighting infections and in finding and attacking cancerous cells.

The weakening of the immune system caused by HIV infection can increase the risk of developing certain types of cancer, such as Kaposi sarcoma, non-Hodgkin lymphoma, and cervical cancer.

You may sometimes see HHV-8 referred to as Kaposi sarcoma-associated herpes virus (KSHV). Like EBV, it’s a type of herpes virus.

Infection with HHV-8 is rare. It’s estimated that less than 10 percent of people in the United States develop an infection.

HHV-8 is mostly spread by saliva, although it can also be transmitted through sexual contact, organ transplants, and blood transfusions.

It causes a rare type of cancer called Kaposi sarcoma. This cancer affects the lining of blood vessels and lymph vessels. HHV-8 can be found in the cells of these tissues.

Normally, the immune system keeps the virus under control. As a result, most people with an infection don’t have any symptoms or develop Kaposi sarcoma.

However, people who have a weakened immune system, due to HIV for example, are at an increased risk for developing Kaposi sarcoma. This is because their immune system may not be able to keep the HHV-8 in check.

According to the National Cancer Institute, there are more than 200 types of HPV. Some types of cause warts to form on the skin, while others cause warts to form on the genitals, throat, or anus. However, HPV infection may not always cause symptoms.

Many types of HPV are spread through skin-to-skin contact during vaginal, anal, or oral sex. Because the virus can spread through skin contact, condom and dental dam use can lower, but not completely prevent, the chances of transmission.

Many people with an HPV infection eventually go on to clear it. However, in some cases long-term HPV infection can lead to cellular changes that can contribute to the development of several cancers, including those of the:

Strains of HPV that can cause these cancers are called high-risk HPVs. There are 14 high-risk strains of HPV, although HPV16 and HPV18 are responsible for most cancers.

Like HIV, HTLV is also a retrovirus. It’s more common outside of the United States in areas such as Japan, the Caribbean, Africa, the Middle East, and South America.

HTLV spreads through blood. Potential means of transmission include:

  • unprotected sexual activity
  • childbirth
  • breastfeeding
  • needling sharing
  • blood transfusions

As a retrovirus, part of the HTLV lifecycle involves integrating viral genes into the those of the host cell. This may affect how the cell grows or expresses its genes and can potentially lead to cancer.

Many people with HTLV infections have no symptoms. However, HTLV infection is associated with an aggressive type of cancer called acute T-cell leukemia/lymphoma (ATL). It’s estimated that 2 to 5 percent of people with the virus will develop ATL.

MCV is a recently discovered virus. Most people contract the virus during childhood and don’t have any symptoms.

It’s unclear how MCV is transmitted, thought experts think skin-to-skin contact is a likely culprit, along with coming into contact with contaminated objects or surfaces.

MCV was first identified in cell samples from a type of cancer called Merkel cell carcinoma, a rare type of skin cancer. It’s now believed that MCV causes nearly all cases of Merkel cell carcinoma.

Oncogenic viruses can cause cancer through different mechanisms, which can include:

  • alteration of cellular genes, either by mutation or by tampering with how genes are expressed
  • suppressing or disrupting the immune system
  • causing long-term inflammation

It’s important to remember that not all viral infections lead to cancer. There are several factors that can influence whether infection with an oncogenic virus will progress to cancer. These can include things like the health of your immune system, genetics, and environment.

Cancer is also a complex disease with many factors that can affect its development. This makes it tricky to say that a virus directly causes cancer. It’s more accurate to think of viruses as one contributing factor in the development of cancer.

There are several things you can do to reduce your risk of contracting an oncogenic virus.


You can avoid two oncogenic viruses by getting vaccinated:

  • The HBV vaccine is recommended for all infants, children, and adolescents. It’s also recommended for adults who may be at risk of HBV infection. The vaccine is given in a series of shots, so you need to get the entire series for full protection.
  • The vaccine Gardasil 9 protects against nine types of HPV, including seven high-risk HPVs. It’s also given in a series and is recommended for children age 11 or 12 or adults up to age 26.

Other tips

In addition to getting vaccinated, you can do several other things to help prevent viral infection, such as:

  • washing your hands frequently, particularly before eating, after using the bathroom, and before touching your face, mouth, or nose
  • not sharing personal items containing saliva or blood, including drinking glasses, toothbrushes, and razors
  • using barrier protection, such as condoms or dental dams, during sexual activity
  • getting regularly screened for HPV if you have a vagina
  • getting regularly screened for HIV and HCV
  • not sharing needles
  • being cautious when getting tattoos or piercings, ensuring that only new, sterile needles are used

Several viruses, known as oncogenic viruses, are associated with cancer. These viruses can cause mutations, affect gene expression, or lead to chronic inflammation.

Keep in mind that having an infection by an oncogenic virus doesn’t mean you’ll develop cancer. It simply means you may have a higher risk than someone who’s never had the infection.