Cancer can, on rare occasions, shrink or vanish entirely without any deliberate treatment. The medical term for this is spontaneous regression (or spontaneous remission), and it has been documented across nearly every type of cancer. The catch is that it is vanishingly uncommon, occurring in roughly one out of every 60,000 to 100,000 cancer cases.1PubMed Central. The spontaneous remission of cancer: Current insights and therapeutic significance That rarity, combined with the difficulty of studying something so unpredictable, means the phenomenon remains poorly understood even after more than a century of case reports. But the patterns researchers have found in these cases are far from random, and they have shaped modern cancer treatments in surprising ways.
Which Cancers Are Most Likely to Regress on Their Own
Spontaneous regression has been recorded in tumors of virtually every tissue type, from lung and breast to colon and bone.2PubMed Central. Spontaneous regression of malignant tumors: Importance of the immune system and other factors (Review) But certain cancers show up in regression case reports far more often than their overall incidence would predict. Over seven decades of documented cases, four groups stand out: melanoma (skin cancer), renal cell carcinoma (kidney cancer), neuroblastoma (a childhood cancer of the adrenal glands), and certain blood cancers including lymphomas and leukemias.3PubMed Central. Why do some cancers suddenly disappear
What do these cancers have in common? They tend to be more “visible” to the immune system than many other tumors. Melanoma cells, for instance, carry a high load of mutations, which means they produce unusual proteins that immune cells can recognize as foreign. Renal cell carcinoma has long been considered an “immunogenic” cancer, one that provokes a strong immune response in at least some patients. Neuroblastoma in infants is a particularly striking case: certain low-risk forms of the disease are now sometimes monitored without treatment because regression rates are high enough that aggressive intervention may do more harm than good. These patterns have steered researchers toward the immune system as the most likely driver behind most cases of spontaneous regression.
The Immune System Connection
The single biggest clue about why cancers occasionally disappear is that regression is usually preceded by an acute infection, a high fever, or some other event that kicks the immune system into high gear.4PubMed Central. Immunity over inability: The spontaneous regression of cancer Case reports going back to the 1800s describe patients whose tumors shrank after bouts of pneumonia, erysipelas (a bacterial skin infection), or other serious febrile illnesses. The connection has been noted so consistently that a large fraction of documented regressions can be linked in time to a strong feverish infection.5PubMed Central. Fever therapy revisited
The working explanation centers on how infection jolts the innate immune system. Under normal conditions, a growing tumor actively suppresses the very immune cells that could destroy it. Dendritic cells and natural killer cells, which ordinarily patrol for damaged or abnormal cells, get inhibited in the tumor’s local environment. A severe infection appears to override that suppression. The intense inflammatory response activates these immune cells broadly and forcefully, and in some patients the reawakened immune system then recognizes and attacks the cancer as well.6International Journal of Cancer and Clinical Research. Common Factors among Some of the Reported Cases of the Spontaneous Remission and Regression of Cancer after Acute Infections Animal studies have supported this picture, showing that rapid inflammatory responses involving granulocytes, a type of white blood cell, can mediate tumor regression as part of innate immunity.7Frontiers in Bioscience-Landmark. Spontaneous Regression of Cancer: Revealing Granulocytes and Oxidative Stress as the Crucial Double-edge Sword
What makes this especially interesting is that a single bout of infection does not always seem to be enough. Some researchers have noted that multiple overlapping acute inflammatory events occurring close together may be particularly effective at breaking through the tumor’s immune suppression, essentially overwhelming the cancer’s defenses through sheer sustained immune activation.6International Journal of Cancer and Clinical Research. Common Factors among Some of the Reported Cases of the Spontaneous Remission and Regression of Cancer after Acute Infections
How Bacteria and Viruses Led to Modern Cancer Immunotherapy
The link between infection and tumor regression is not just an academic curiosity. It directly inspired one of the earliest forms of cancer immunotherapy. In the 1890s, a New York surgeon named William Coley began deliberately injecting cancer patients with heat-killed bacteria, a mixture of Streptococcus pyogenes and Serratia marcescens that became known as Coley’s toxin. The idea was to provoke the kind of fierce immune response that had been observed to shrink tumors in patients who happened to catch infections. The approach produced documented regressions in sarcomas, carcinomas, lymphomas, melanomas, and myelomas.8PubMed Central. Spontaneous regression of tumour and the role of microbial infection–possibilities for cancer treatment
Coley’s work eventually fell out of favor as radiation and chemotherapy took center stage, but his core insight survived. The most direct descendant in modern clinical practice is the use of BCG vaccine (Bacillus Calmette-Guérin, originally developed for tuberculosis) to treat superficial bladder cancer. BCG is instilled directly into the bladder, where it provokes a local immune response that attacks the cancer cells. It remains a standard first-line treatment for certain bladder tumors, and it is the most common clinical use of a microorganism against cancer.8PubMed Central. Spontaneous regression of tumour and the role of microbial infection–possibilities for cancer treatment The broader principle, that the immune system can be pushed to destroy cancer if you give it the right provocation, is also the foundation of modern checkpoint inhibitor drugs, which work by removing the brakes that tumors place on immune cells.
The history matters because it reframes spontaneous regression from a mysterious fluke into an extreme natural example of something oncologists are now trying to engineer on purpose. The rarity of spontaneous regression is not because the immune system lacks the firepower to destroy cancer. It is because, in most patients, the circumstances that would fully unleash that firepower never come together.
Other Triggers Beyond Infection
Infection and fever dominate the case reports, but they are not the only events that precede spontaneous regression. Several other triggers have been documented, each suggesting a different pathway.
Biopsy and Surgical Trauma
A number of case reports describe tumors regressing shortly after a biopsy, even when the biopsy removed only a tiny sample and no further treatment was given. A case of primary squamous cell lung cancer that regressed after tissue biopsy led researchers to propose that the physical trauma of the biopsy procedure triggered a local immune response strong enough to attack the remaining tumor.9PubMed Central. Spontaneous regression of a primary squamous cell lung cancer following biopsy: a case report A similar pattern was seen in a breast carcinoma that regressed after core needle biopsy. The proposed mechanism is what researchers call immunogenic cell death: the surgical trauma attracts immune cells to the area, and those immune cells then encounter and destroy cancer cells that they might otherwise have ignored.10PubMed Central. Breast carcinoma with spontaneous regression after needle biopsy: a case report and literature review
Hormonal Shifts
Some cancers depend heavily on specific hormones to grow. Breast and prostate cancers are the best-known examples. In these cases, changes in a patient’s hormonal environment, whether from natural shifts like menopause or from unrelated medical events, can starve the cancer of the signals it needs to survive. Research going back decades has established that both removing essential hormones and flooding the system with interfering hormones can cause hormone-dependent cancer cells to die.11Science. Endocrine-Induced Regression of Cancers This is the basis of modern hormone therapy for breast and prostate cancer, but it also explains some cases that might look “spontaneous” when a patient’s hormonal balance shifts for reasons unrelated to their cancer treatment.
Disrupted Blood Supply
Tumors need blood flow to grow. Cases of spontaneous regression have been reported after events that severely reduced oxygen delivery to a tumor, including pneumonia, pulmonary edema, hemorrhage, low hemoglobin levels, and even blockage of arteries feeding the tumor. All of these share the feature of starving the tumor of oxygen and nutrients.12International Journal of Cancer and Clinical Research. Proposed Biology of the Spontaneous Remission of Cancer The vascular angle is interesting because modern cancer treatments already exploit this idea: drugs that block the growth of new blood vessels into tumors are a standard part of treatment for several cancer types.
When It Might Not Be Regression at All
Not every apparent case of spontaneous regression is what it seems. One important caveat is diagnostic misattribution. A patient diagnosed with cancer whose tumor later vanishes could, in some cases, have been misdiagnosed in the first place. This is especially relevant for cancers that are difficult to distinguish from benign conditions using imaging alone. Pancreatic cancer is a well-documented example: benign pancreatic tumors, insulinomas, and autoimmune pancreatitis can all mimic pancreatic cancer on scans, and if one of these conditions resolves on its own, it could be mistakenly counted as a case of spontaneous cancer regression.13PubMed Central. Spontaneous regression of pancreatic cancer: real or a misdiagnosis?
This is not a trivial concern. The rarity of spontaneous regression means even a small number of misdiagnosed cases can significantly inflate the reported rate for certain tumor types. Researchers reviewing the literature have repeatedly emphasized that many historical case reports lack the rigorous biopsy-confirmed diagnoses that would rule out misidentification. When you read about a “spontaneous regression” in an older case report, there is often no way to be certain the original diagnosis was correct. The most convincing modern cases are those with tissue biopsy confirming malignancy before the regression began.
COVID-19 and Unexpected Tumor Shrinkage
The COVID-19 pandemic added a new and unexpected chapter to the story of spontaneous regression. Researchers compiled sixteen case reports of patients whose cancers shrank or disappeared following COVID-19 infection, and an additional two cases after anti-SARS-CoV-2 vaccination.14PubMed Central. Spontaneous cancer remission after COVID-19: insights from the pandemic and their relevance for cancer treatment The cases spanned different cancer types and included patients who had no ongoing cancer treatment at the time of their infection.
Two mechanisms have been proposed. One is the “oncolytic hypothesis,” which suggests that the virus itself may have directly infected and destroyed tumor cells, similar to how oncolytic viruses are being engineered for cancer therapy. The other is the “priming hypothesis,” which is essentially the same immune-activation story described earlier: the massive inflammatory response triggered by COVID-19 reactivated dormant anti-tumor immunity. Researchers were careful to note that COVID-19 was also associated with cancer progression in some patients, so the immune response is clearly a double-edged sword. Still, these cases reinforce the broader principle that intense immune activation can, in some individuals, tip the balance against an existing cancer.
Lessons from Other Species
Spontaneous regression is not unique to humans. Dogs, in particular, have provided valuable research material. One well-documented case involved an Irish Wolfhound whose lung metastases from a bone cancer (tibial fibrosarcoma) disappeared after the affected leg was amputated, with no further treatment targeting the lung tumors.15Veterinary Radiology. SPONTANEOUS REGRESSION OF PULMONARY METASTASES IN A DOG
But the most remarkable example from the animal world is the canine transmissible venereal tumor (CTVT), a cancer that spreads between dogs through physical contact. CTVT is bizarre: it is a living cell line thousands of years old that passes from dog to dog like a parasite. Yet in most cases, after an initial period of growth, the tumor spontaneously regresses. Researchers studying this process found that regression follows a clear sequence: first the innate immune system activates, then immune cells infiltrate the tumor, the cancer cells stop dividing, and the tissue repairs itself.16Cancer Cell. Systematic Analysis of Canine Transmissible Venereal Tumor Regression CTVT offers something that human spontaneous regression almost never does: a repeatable, studyable model of the immune system defeating an established cancer. The stepwise process observed in CTVT broadly mirrors what researchers suspect happens in the rare human cases.
Epigenetic Switches and Built-In Brakes
Not all proposed mechanisms for spontaneous regression revolve around the immune system. Some cancers may regress because of internal changes in how their genes are regulated. Retinoblastoma, an eye cancer that occurs in young children, is a notable case. Researchers have found evidence that changes in the methylation pattern of the RB gene, the tumor suppressor gene whose loss drives this cancer, may play a role in both the development and the spontaneous regression of some retinoblastoma tumors.17PubMed. Epigenetic changes may contribute to the formation and spontaneous regression of retinoblastoma In other words, the same type of chemical tagging that silenced a protective gene and allowed the cancer to start growing could, under certain conditions, reverse itself and reactivate that gene, switching the cancer off.
There is also a broader built-in defense mechanism that may contribute. Human cells have a natural limit on how many times they can divide, enforced partly by the shortening of telomeres, the protective caps on the ends of chromosomes. Most cancer cells bypass this limit by activating an enzyme called telomerase, which maintains their telomeres indefinitely. But not all cancers manage this trick efficiently. The evolutionary repression of telomerase in human tissues may itself function as an anticancer safeguard, and in tumors that fail to fully activate telomerase, natural telomere shortening could eventually halt their growth.18PubMed Central. Role of Telomeres and Telomerase in Aging and Cancer Whether this mechanism accounts for any documented cases of spontaneous regression in adult cancers is speculative, but it illustrates that cancer cells are not invincible. They carry vulnerabilities that, under the right circumstances, their own biology can exploit.
Why This Should Not Change How You Think About Treatment
The existence of spontaneous regression is scientifically fascinating, and it has generated real insights into how the immune system interacts with cancer. But its practical message for anyone diagnosed with cancer is straightforward: do not count on it. At roughly one in 60,000 to 100,000 cases, the odds are comparable to being struck by lightning, and many of those documented cases involved partial regressions rather than complete cures. Some regressions were temporary, with the cancer returning months or years later.
The danger of overstating spontaneous regression is that it feeds into a narrative promoted by some alternative medicine practitioners, the idea that the body can “heal itself” of cancer if given the right diet, mindset, or supplement. The evidence simply does not support that framing. The documented triggers for regression are severe biological events: life-threatening infections, significant surgical trauma, major hormonal disruptions. These are not wellness interventions. They are physiological crises that happen to, in extremely rare cases, produce a beneficial side effect.
Where spontaneous regression does matter practically is in the research lab. Every confirmed case is a natural experiment showing that the human body can, under the right conditions, eliminate an established cancer. Understanding exactly what those conditions are has already helped produce some of the most promising cancer treatments of the past two decades, including checkpoint inhibitor immunotherapy and oncolytic virus therapy. The goal is not to wait for spontaneous regression to happen but to figure out what the immune system does in those rare cases and make it happen reliably, on purpose, in every patient who needs it.
The Microbiome Angle
A more recent thread of research connects the gut microbiome to cancer immunity and, by extension, to the conditions that might favor spontaneous regression. The observation that bacteria and cancer have a relationship is not new; the very first clinical report of a tumor regressing after bacterial infection dates to 1868, when William Busch documented tumor shrinkage in a patient with a Streptococcus pyogenes infection.19Science. The microbiome and human cancer What has changed is the recognition that the trillions of microbes living inside us are not just passive bystanders. They actively shape immune function, including the types of immune responses that determine whether cancer grows or is held in check.
Modern immunotherapy research has shown that patients with certain gut microbial profiles respond better to checkpoint inhibitor drugs than patients with less diverse microbiomes. The connection to spontaneous regression is circumstantial for now, but it raises an intriguing possibility: some of the variability in how different patients’ immune systems handle cancer may trace back, at least in part, to the microbial ecosystems they carry. If a severe infection reshuffles that ecosystem at the same time it fires up the immune system, the combined effect could push a patient across the threshold from tumor tolerance to tumor rejection. That remains speculative, but it is the kind of question that ongoing microbiome research is well positioned to answer.