However, though cancer itself is not contagious, some infections (which are contagious) can increase a person’s risk of developing cancer. Cancer may run in families, but instead of being transmitted, this risk is related to genetic traits (a genetic predisposition) or common exposures that increase risk.
Contagiousness and Cancer
Since cancer can be contagious in some animals, you may be wondering why it is not contagious in humans. This is because cancer cells cannot live outside the body, and the immune system recognizes cancer cells entering the body.
In an unethical experiment conducted in the 1950s and 1960s, Dr. Chester Southam injected live cancer cells into unknowing people to see if he could “cause” cancer. With only one exception, the recipient’s immune system fought off the cancer cells before they passed beyond the nodule stage.
The lack of contagiousness of cancer is also better understood when looking at how cancer develops.
Cancer cells arise after a series of mutations (in genes that control the growth of the cell) lead to uncontrolled growth. Even when genetic damage occurs, the human body has genes (such as tumor suppressor genes) that code for proteins to repair damaged DNA or eliminate damaged cells.
Cancer Risk in Families
Genetics plays a role in cancers that might seem contagious (they run in families), but despite this clustering of cancers, the cancers are not directly passed from one person to another.
A genetic predisposition to cancer does not mean a person will get cancer. Hereditary cancer accounts for roughly 10% of cancers overall (the influence of genetics can vary by type). Many gene mutations associated with cancer (such as BRCA) occur in tumor suppressor genes.
Tumor suppressor genes code for proteins that repair DNA that has been damaged or instead eliminate the cell before it becomes a cancer cell. In this case, having the mutated gene does not cause cancer but interferes with the body’s ability to repair damaged cells that environmental exposures have damaged.
Even without a genetic predisposition, cancer may appear to cluster in families. This may be due to shared lifestyle habits (such as smoking or dietary habits) and exposure to similar environmental carcinogens, such as radon exposure in the home. Cancers may also occur due to exposure to the virus (such as hepatitis B) transmitted between family members.
Organ Transplants and Cancer Risk
As noted above, the immune system destroys cancer cells from another person that enters our bodies. However, as an exception to this general rule, there have been cases of cancer being transmitted from one person to another via organ transplant, and it’s thought that transfusion-related cancer may occur in roughly 3 out of 5,000 transplant recipients.
With organ transplants, two factors contribute to this risk. One is that instead of just a few cancer cells (such as with a needle stick), a large volume of tumor cells are implanted in a person (from a mass in the transplanted organ). In addition, these people are usually severely immunocompromised due to the medications used to prevent rejection.
Cancer Transmission During Pregnancy
There are a few reported cases of cancer transmission during pregnancy, which may occur in three ways.
From the mother to the baby: While tumors may spread to the placenta, the placenta usually prevents cancer cells from reaching the baby. The chance of cancer being transmitted (1 in 1,000 pregnant people are thought to have cancer) is estimated at only 0. 000005%. Transmission is most common with leukemia/lymphomas and melanoma. Twin to twin transmission of leukemia: Again, transmission is very rare but may occur at times. Choriocarcinoma: Choriocarcinoma is a rare tumor that arises in the placenta. The tumor may spread to both the mother and the baby and is the only case of serial transmission of cancer (from the placenta to the mother and then from the mother to recipients of organs donated by that mother).
Infections That Increase Cancer Risk
Some infections passed from person to person are thought to lead to cancer. In these cases, however, it’s not cancer per se that is contagious, but rather the infection that may or may not (and in most cases does not) lead to cancer.
Infections with these microorganisms are common, whereas the cancers that arise from the infections are not. In addition, most cancers are multifactorial in origin (have many causes), and other factors such as exposure to carcinogens, immunosuppression, genetic factors, lifestyle, and more may combine with the infection to induce cancer.
Infections may lead to cancer in different ways. For example, some may cause inflammation that leads to cancer (due to increased cell division of cells involved in repair), whereas others may cause immunosuppression. Yet others may damage DNA (cause mutations) directly.
Viruses associated with cancer include:
Human papillomavirus (HPV): HPV is the most common sexually transmitted infection and has been linked with cervical cancer, anal cancer, penile cancer, vaginal cancer, and head and neck cancers. In most cases, infection with HPV goes away on its own, but when persistent, it may lead to inflammation and cancer. Not all strains of HPV are linked with cancer. Hepatitis B and hepatitis C virus: Both hepatitis B and C are associated with liver cancer and are the most significant cause worldwide. Epstein Barr virus (EBV): EBV is best known for being the cause of mononucleosis, though it has been linked to several cancers. It is also associated with Burkitt’s lymphoma, gastric carcinoma, Hodgkin’s lymphoma, and nasopharyngeal carcinoma. While 90% to 95% of people are infected globally, about 1% of those develop cancer. HIV/AIDS: Several types of cancer are associated with HIV/AIDS, related to immunosuppression. Human herpesvirus Type 8 (HHV-8) or Kaposi sarcoma herpes virus most commonly leads to Kaposi’s sarcoma in people with HIV. Human T-lymphotropic virus-1 (HTLV-1): HTLV-1 is associated with some leukemias and lymphomas, but while the infection is relatively common, cancers are not. Merkel cell polyomavirus: The Merkel cell polyomavirus is very common worldwide but rarely leads to a type of skin cancer called Merkel cell carcinoma.
Bacteria associated with cancer include:
H. pylori: H. pylori infection is associated with stomach cancer and peptic ulcer disease.
Parasites associated with cancer include:
Liver flukes: Two different liver flukes are linked with bile duct cancer and are found primarily in East Asia. Schistosomiasis: The worm that causes this disease is associated with bladder cancer.
In addition to these specific organisms, microorganisms on or in our bodies may be associated with an increased or decreased risk of cancer. For example, the microbiome of the skin (normal bacteria that live on the skin) may be related to the development of skin cancer, and good gut bacteria may lower lymphoma risk.
How to Prevent the Spread of Infections
HPV can be transmitted sexually, and hepatitis B and C, as well as HIV, can be transmitted sexually and through contact with blood. Hepatitis B is spread much more easily than HIV, and even sharing a toothbrush could lead to transmission.
Safer sex includes the use of condoms and more. Blood precautions are necessary with hepatitis B, C, and HIV. With hepatitis B, immunization is the best way to prevent the disease.
For those undergoing chemotherapy, precautions may be made to protect both partners.
People With Cancer
People receiving chemotherapy should use a condom, as becoming pregnant with some chemotherapy drugs is associated with congenital disabilities. In addition, oral, vaginal, and anal sex should be avoided if either partner has open sores. If your white blood count is very low (chemotherapy-induced thrombocytopenia), sex should be postponed until your white count is higher. Oncologists differ on the count they consider too low, but an absolute neutrophil count of 500 or less is sometimes used as a cutoff. The nadir period is when white blood cell counts are usually the lowest. Both partners should wash their hands (or use hand sanitizer) before sex, and genitals should be washed before oral sex. Women should urinate shortly after sex to reduce the risk of a bladder infection. Water-based lubricants should be used to avoid abrasion and consequent infection risk. Sex should also be avoided if your platelet count is low (chemotherapy-induced thrombocytopenia), usually defined as a platelet count of less than 50,000 due to the risk of bleeding. Indeed, you should avoid close contact with your partner if they are ill.
Loved Ones of Those With Cancer
Chemotherapy drugs may be present in saliva, semen, and vaginal secretions. Your loved one’s oncologist may recommend avoiding sex shortly after a chemotherapy infusion, but this can vary. People who are or may be pregnant should talk to their partner’s oncologist about possible exposure and timing. With some types of radiation, such as internal radiation (brachytherapy) or radioactive iodine treatment, your radiation oncologist may recommend avoiding close contact, especially if you are pregnant.
Cancer cannot be transmitted by touching, kissing, or sex (except for a few precautions). However, it’s usually fine to be intimate, and in many situations, intimacy is advised. A few precautions are important for those who have infections linked with cancer and those living with cancer.
Summary
Cancer is not contagious. Environmental factors or genetic predisposition can make it more likely that an individual develops cancer. Certain infections can increase the likelihood of cancer. Those diagnosed with cancer or partners of those with cancer need to take additional precautions to prevent the spread of infections through intimate contact.
A Word From Verywell
Cancer is not contagious, and you do and should not stay away from friends or loved ones with cancer. In fact, offering your support and being near is more important than ever, and some studies have even found that better social support is linked with improved survival.
If your loved one may have a virus associated with an infectious disease, learn about the disease and any precautions you can take. You should also talk with your oncologist about any risk to you or your partner regarding intimacy during treatment.
