Many wonder if cancer is contagious like a cold or flu. For the vast majority of cases, human cancer is not. This article explains why cancer generally does not spread from person to person and addresses extremely rare exceptions.
Understanding Cancer
Cancer is a disease of the body’s own cells, characterized by uncontrolled growth and division. Normal cells grow and divide only when needed, but cancerous cells disregard these signals, multiplying without restraint and often forming tumors.
This uncontrolled proliferation stems from genetic mutations within the cells’ DNA. These mutations can occur spontaneously as cells divide, be inherited, or result from exposure to environmental factors like tobacco smoke or ultraviolet radiation. Unlike infectious diseases caused by external pathogens, cancer originates from internal cellular changes.
Why Human Cancers Are Not Contagious
The primary reason human cancers are not contagious lies in the sophisticated defense mechanisms of the immune system. The immune system recognizes and destroys foreign invaders. If cancer cells from one person were to enter another healthy individual, the recipient’s immune system would identify these cells as foreign and eliminate them.
This rejection process is similar to how the body rejects an incompatible organ transplant. Each person possesses unique cellular markers, known as major histocompatibility complex (MHC) molecules, which act like identity tags. When foreign cells with different MHC markers enter the body, the immune system recognizes them as “non-self” and destroys them. Cancer cells also lack the specific mechanisms infectious agents use to spread efficiently, such as actively invading new cells or replicating independently outside the body.
Extremely Rare Cases of Cancer Transmission
While cancer is generally not transmissible between humans, extremely rare circumstances allow cancer cells to be transferred. One scenario involves organ or tissue transplantation. If a donor has undiagnosed cancer, cells can be transferred to the recipient with the transplanted organ. This risk is very low, estimated at around 0.2% of donor-derived cancer transmission, due to rigorous screening. However, transplant recipients are often on immunosuppressive medications to prevent rejection, which can weaken their ability to fight off any transferred cancer cells.
Another rare instance is mother-to-fetus transmission during pregnancy. This occurs when cancer cells from a pregnant woman cross the placenta and enter the developing fetus. Though documented cases are few, the baby’s immune system usually fights off these foreign cells. Most recorded cases involve specific types of cancer, such as melanoma or leukemia.
Transmissible cancers exist in some animal species, distinct from human cancers. Examples include Canine Transmissible Venereal Tumor (CTVT) in dogs and Devil Facial Tumour Disease (DFTD) in Tasmanian devils. CTVT spreads through living cancer cell transfer, typically during mating, while DFTD transmits via biting during social interactions. These animal cancers are unique because their cells can survive and grow as “allografts” in genetically diverse individuals within their species.
Debunking Common Fears
Concerns about cancer contagiousness are understandable, but cancer is not an infectious disease in humans. Everyday interactions with a person who has cancer, such as hugging, kissing, or sharing food, pose no risk of cancer transmission. Cancer cells cannot survive or replicate independently outside the human body or establish themselves in another healthy individual.
Cancer is not like the common cold or flu, which spread through air or direct contact. Dispelling these misconceptions is important, as fear can lead to social isolation for individuals undergoing cancer treatment. Supporting and empathy for cancer patients is important, as they are not a source of infection.