Cancer occurs when cells in the body grow and divide uncontrollably, forming tumors or spreading to other parts of the body. This happens due to changes (mutations) in the DNA of cells that disrupt normal controls on cell growth, division, and death. These mutations can turn on oncogenes (genes that promote cell growth) or turn off tumor suppressor genes (which normally prevent excessive growth). 32 While environmental factors like smoking, radiation, chemicals, and viruses play a significant role in many adult cancers, they’re not the sole cause—random errors during cell division and inherited factors also contribute.

Environmental factors are indeed important for many cancers, but childhood cancers often arise differently. Most occur due to DNA mutations that happen very early in a child’s life, sometimes even before birth during fetal development. 0 These are typically random errors in cell division rather than accumulated environmental exposures over time, as children haven’t had long-term exposure to carcinogens like tobacco or UV radiation. 1 Inherited genetic mutations account for about 5-10% of childhood cancers, while prenatal factors (like certain infections or rare environmental exposures in utero) can play a role in others. 5 Unlike adult cancers, lifestyle isn’t a major factor, and it’s not caused by anything the parents or child did. 1

Yes, gene mutations are the underlying cause of cancer, as they alter how cells behave. What causes these mutations? Some are inherited from parents (germline mutations), but these make up only a small fraction (5-10%) of cancers. 38 Most are acquired during a person’s lifetime, often from random mistakes when cells copy their DNA during division—this accounts for about two-thirds of cancer-causing mutations. 25 Other triggers include environmental exposures (e.g., UV radiation from the sun causing skin cancer, chemicals in tobacco smoke leading to lung cancer), viruses (like HPV for cervical cancer or hepatitis for liver cancer), chronic inflammation, and aging, which increases the chance of errors accumulating. 28 36

The autoimmune function of the immune system does play a role in cancer. The immune system normally surveils the body for abnormal cells, including precancerous ones, and destroys them through mechanisms like T cells and natural killer cells. 41 In cancer, tumors often evade this by inducing immune tolerance—essentially tricking the system into ignoring them, similar to how the body tolerates its own cells to prevent autoimmunity. 40 Autoimmunity (where the immune system attacks healthy tissues) and cancer can be seen as opposites: autoimmunity involves overactive responses to self, while cancer thrives on underactive responses to malignant cells. 40 42 Chronic inflammation from autoimmune conditions can also promote DNA damage and increase cancer risk in some cases. 16

The relationship between autoimmune diseases and cancer risk is complex and not simply inverse—people with autoimmune diseases don’t uniformly have lower cancer rates, and vice versa. Overall, having an autoimmune disease often increases the risk of certain cancers by about 11%, due to chronic inflammation, immune dysregulation, or immunosuppressive treatments. 11 15 For example, rheumatoid arthritis (RA) is linked to higher risks of lymphoma and lung cancer but lower risks of breast and uterine cancers. 14 Psoriasis may slightly increase breast cancer risk, while systemic lupus erythematosus (SLE) raises risks for hematologic and some gynecologic cancers. 14 15 Conversely, some genetic factors that predispose to autoimmunity (like certain PTPN22 variants) may lower overall cancer incidence by enhancing immune surveillance. 10 Cancer treatments like immunotherapy can sometimes trigger autoimmune-like side effects, highlighting the shared pathways. 48 Most people with autoimmune diseases won’t develop cancer, but regular screening is recommended based on specific risks. 11