Viruses are tiny particles of genetic code. Some viruses can insert their own genes into host cells and then use the cells as factories to make more viruses. This process can change a cell’s genes and, in rare cases, make the cell become cancerous.

Some people with a history of viral infections are more likely to develop cancer. In one study, 52 percent of breast cancer tissue samples had DNA markers for viruses that are linked to breast cancer.

Viruses Linked to Breast Cancer

Several types of viruses may increase breast cancer risk. Some people with breast cancer have been infected with one of these viruses, and some people have multiple viruses in their breast cancer tissue. Scientists don’t know for sure that all of these viruses cause breast cancer, but suspicions persist.

However, not all breast cancer cases are caused by viruses. Some people with breast cancer don’t have any known viral infections that could increase risk.

Mouse Mammary Tumor Virus

In spite of its name, mouse mammary tumor virus (MMTV) can infect mice, dogs, and cats as well as humans. MMTV can be passed between people through saliva. It may also be spread through the saliva of dogs or through mouse or rat feces.

MMTV can also be transmitted from mother to child through breast milk. However, studies have found that breastfeeding doesn’t impact a child’s risk of breast cancer once they get older.

Humans can also get the virus directly from a certain species of mice: Mus domesticus. Rates of breast cancer are highest in areas where these mice are found. However, many other factors (such as higher income level) also play a role in higher breast cancer rates in these areas.

The MMTV virus is more often found in breast cancer cells compared to normal breast cells. An estimated 40 percent of people with breast cancer have MMTV genes in their cancer cells, although the exact number varies widely in different studies. MMTV is also detected in normal breast tissue samples about 24 percent of the time.

Researchers have occasionally found MMTV in other types of cancers, especially cancers that are affected by hormones. These include liver cancer, lung cancer, ovarian cancer, endometrial cancer, prostate cancer, and skin cancer.

Human Papillomavirus

Human papillomavirus (HPV) infections are a major cause of cervical cancer. The virus has also been linked to cancers that develop in the mouth and throat, penis, vagina, vulva, and anus.

Most people contract HPV over the course of their lifetime, and there are more than 150 types of the virus. Some types are spread through sexual contact, while others can be spread through touch. HPV does not usually cause any symptoms, although many of them can cause warts, including genital warts.

Certain types of HPV come with a higher risk of developing cancer. However, most people with HPV infections will never be diagnosed with cancer.

Several studies have linked HPV with breast cancer. One study found that people with breast cancer are more than six times as likely to have a history of HPV infection. Women with HPV-associated cervical cancer have a higher chance of being diagnosed with HPV-associated breast cancer. However, this virus is not always responsible for breast cancer — many people with breast cancer have never been infected with HPV.

Epstein-Barr Virus

Epstein-Barr virus (EBV) is a very common virus. Globally, more than 9 out of 10 people have been infected with EBV. These infections often occur during childhood.

EBV infections cause infectious mononucleosis, also known as mono. People who have an active infection may experience fever, fatigue, a sore throat, or rash. In some cases, EBV infections feel like a mild cold. Children often don’t experience any symptoms from these infections.

EBV infections have been linked with several different cancer types, including breast cancer. In one large review that combined data from 30 studies, people who had been infected with EBV were nearly five times more likely to develop breast cancer than those without this virus. EBV infections also lead to a higher chance of being diagnosed with:

• Nasopharyngeal cancer (cancer in the upper part of the throat, behind the nose)
• Lymphoma, including Burkitt lymphoma and Hodgkin lymphoma
• Stomach cancer

EBV spreads through things like sneezing, coughing, and sharing a cup or silverware. Once you have EBV, there are no treatments to remove it. However, the vast majority of people who have been infected with EBV do not develop cancer.

Bovine Leukemia Virus

Bovine leukemia virus (BLV) is a virus that can cause leukemia, a blood cancer, in cows. BLV is similar to another cancer-causing virus, human T-cell lymphotrophic virus type 1. Humans can possibly become infected with BLV after drinking milk from infected cows.

In one study, almost 6 out of 10 people with breast cancer had been infected with BLV, but only about 3 out of 10 people without breast cancer were infected.

How Do Viruses Lead To Breast Cancer?

Researchers are still learning how different types of viruses can create changes that lead to cancer. They have discovered several possible explanations.

When viruses infect cells, they insert their own genes into the cell’s genes. This process can sometimes lead to cancer-causing gene changes. For example, cells have many genes that encourage cells to grow and divide. In most cells, these genes are turned off or regulated after a person goes through development. In virus-infected cells, however, the inserted material may activate these genes, causing the cell to grow out of control and form a tumor.

Viral infections can also cause mutations in genes linked to breast cancer. For example, HPV can cause mutations in the BRCA genes, which are often responsible for breast cancer development.

Viruses may contain proteins that have the potential to transform a normal cell into a cancer cell. For example, MMTV makes proteins that can cause cells to undergo cancerous changes. HPV proteins can destroy p53 — an important protein found in cells that acts as a brake, preventing cells from growing too quickly.

Hormones in the body may also activate MMTV viral particles located within cells, making them more likely to turn a cell cancerous.

In some cases, multiple virus types can work together to lead to cancer. For example, proteins made by HPV can work together with EBV proteins to make cells more cancerous. HPV can also shut off defenses in a cell that make it easier for MMTV to cause cancer.

Preventing Viral Infections

Some viral infections can be prevented with vaccines, which help train the immune system to fight off viruses.

Researchers have developed an HPV vaccine that can help prevent people from being infected with this virus. This vaccine can be given to children, adolescents, or young adults. The HPV vaccine can protect against more than 90 percent of cases of HPV-linked cancers.

Some researchers are studying vaccines for MMTV. Their experiments have been able to prevent tumors from forming in mice. One day, an MMTV vaccine may be available for humans.

There is not yet a vaccine for EBV. A person can reduce their chances of becoming infected with EBV by avoiding contact with people who are sick. However, this virus is hard to prevent, and most people contract EBV infections as children.

BLV vaccination is available for cows. Preventing animals from getting this virus helps stop it from being transmitted through cow’s milk. These vaccines may help lower rates of human breast cancer.

Talk With Others Who Understand

MyBCTeam is the social network for people with breast cancer and their loved ones. On MyBCTeam, more than 54,300 members come together to ask questions, give advice, and share their stories with others who understand life with breast cancer.

Are you living with breast cancer? Share your experiences in the comments below, or start a conversation by posting on your Activities page.