Cancer: What is it and how does it spread?

According to the U.S. National Cancer Institute, cancer is a term for diseases in which abnormal cells divide without control and can invade nearby tissues. Cancer cells can also spread to other parts of the body through the blood and lymph systems. On this page we look at the different types and causes of cancer as well as metastasis.

There are many types of cancer.

The National Cancer Institute tells us there are more than 100 types of cancer. Types of cancer are usually named for the organs or tissues where the cancers form. For example, lung cancer starts in the lung, and brain cancer starts in the brain. Cancers also may be described by the type of cell that formed them, such as an epithelial cell or a squamous cell.

Here are some categories of cancers that begin in specific types of cells:

Carcinoma
Carcinomas are the most common type of cancer. They are formed by epithelial cells, which are the cells that cover the inside and outside surfaces of the body. There are many types of epithelial cells, which often have a column-like shape when viewed under a microscope.
Carcinomas that begin in different epithelial cell types have specific names:

Adenocarcinoma is a cancer that forms in epithelial cells that produce fluids or mucus. Tissues with this type of epithelial cell are sometimes called glandular tissues. Most cancers of the breast, colon, and prostate are adenocarcinomas.

Basal cell carcinoma is a cancer that begins in the lower or basal (base) layer of the epidermis, which is a person’s outer layer of skin.

Squamous cell carcinoma is a cancer that forms in squamous cells, which are epithelial cells that lie just beneath the outer surface of the skin. Squamous cells also line many other organs, including the stomach, intestines, lungs, bladder, and kidneys. Squamous cells look flat, like fish scales, when viewed under a microscope. Squamous cell carcinomas are sometimes called epidermoid carcinomas.

Transitional cell carcinoma is a cancer that forms in a type of epithelial tissue called transitional epithelium, or urothelium. This tissue, which is made up of many layers of epithelial cells that can get bigger and smaller, is found in the linings of the bladder, ureters, and part of the kidneys (renal pelvis), and a few other organs. Some cancers of the bladder, ureters, and kidneys are transitional cell carcinomas.

Sarcoma
Sarcomas are cancers that form in bone and soft tissues, including muscle, fat, blood vessels, lymph vessels, and fibrous tissue (such as tendons and ligaments).

Osteosarcoma is the most common cancer of bone. The most common types of soft tissue sarcoma are leiomyosarcoma, Kaposi sarcoma, malignant fibrous histiocytoma, liposarcoma, and dermatofibrosarcoma protuberans.

Leukemia
Cancers that begin in the blood-forming tissue of the bone marrow are called leukemias. These cancers do not form solid tumors. Instead, large numbers of abnormal white blood cells (leukemia cells and leukemic blast cells) build up in the blood and bone marrow, crowding out normal blood cells. The low level of normal blood cells can make it harder for the body to get oxygen to its tissues, control bleeding, or fight infections.
There are four common types of leukemia, which are grouped based on how quickly the disease gets worse (acute or chronic) and on the type of blood cell the cancer starts in (lymphoblastic or myeloid). Acute forms of leukemia grow quickly and chronic forms grow more slowly.

Lymphoma
Lymphoma is cancer that begins in lymphocytes (T cells or B cells). These are disease-fighting white blood cells that are part of the immune system. In lymphoma, abnormal lymphocytes build up in lymph nodes and lymph vessels, as well as in other organs of the body.
There are two main types of lymphoma:

Hodgkin lymphoma – People with this disease have abnormal lymphocytes that are called Reed-Sternberg cells. These cells usually form from B cells.

Non-Hodgkin lymphoma – This is a large group of cancers that start in lymphocytes. The cancers can grow quickly or slowly and can form from B cells or T cells.

Multiple Myeloma
Multiple myeloma is cancer that begins in plasma cells, another type of immune cell. The abnormal plasma cells, called myeloma cells, build up in the bone marrow and form tumors in bones all through the body. Multiple myeloma is also called plasma cell myeloma and Kahler disease.

Melanoma
Melanoma is cancer that begins in cells that become melanocytes, which are specialized cells that make melanin (the pigment that gives skin its color). Most melanomas form on the skin, but melanomas can also form in other pigmented tissues, such as the eye.

Brain and Spinal Cord Tumors
There are different types of brain and spinal cord tumors. These tumors are named based on the type of cell in which they formed and where the tumor first formed in the central nervous system. For example, an astrocytic tumor begins in star-shaped brain cells called astrocytes, which help keep nerve cells healthy. Brain tumors can be benign (not cancer) or malignant (cancer).

Other Types of Tumors

Germ Cell Tumors
Germ cell tumors are a type of tumor that begins in the cells that give rise to sperm or eggs. These tumors can occur almost anywhere in the body and can be either benign or malignant.

Neuroendocrine Tumors
Neuroendocrine tumors form from cells that release hormones into the blood in response to a signal from the nervous system. These tumors, which may make higher-than-normal amounts of hormones, can cause many different symptoms. Neuroendocrine tumors may be benign or malignant.

Carcinoid Tumors
Carcinoid tumors are a type of neuroendocrine tumor. They are slow-growing tumors that are usually found in the gastrointestinal system (most often in the rectum and small intestine). Carcinoid tumors may spread to the liver or other sites in the body, and they may secrete substances such as serotonin or prostaglandins, causing carcinoid syndrome.

Identifying your cancer sub-type.

Many of the genes that contribute to cancer development fall into broad categories:

Tumor suppressor genes – BRCA1, BRCA2, p53 or TP53.
These are protective genes but they can mutate and turn into cancer cells.
Oncogenes. – HER2, RAS
These turn cells cancerous. Mutations in these genes are not inherited.
DNA repair genes – BRCA1, BRCA2, and p53 
These fix mistakes made when DNA is copied. Many of them function as tumor suppressor genes. If a person has an error in a DNA repair gene, mistakes remain uncorrected. Then, the mistakes become mutations. These mutations may eventually lead to cancer, particularly mutations in tumor suppressor genes or oncogenes. Mutations in DNA repair genes may be inherited or acquired. 

The beginning – Carcinogenesis

Carcinogenesis is the process in which normal cells turn into cancer cells.

The list below includes the most-studied known or suspected risk factors for cancer. Although some of these risk factors can be avoided, others—such as growing older—cannot.

• Age
• Alcohol
• Cancer-Causing Substances
• Chronic Inflammation
• Diet
• Hormones
• Immunosuppression
• Infectious Agents
• Obesity
• Radiation
• Overexposure to Sunlight
• Tobacco

For up-to-date information on cancer please visit the National Cancer Institute

Cancers caused by viruses, bacteria and parasites

This is not a complete list.

CDC researchers link cancer cells from parasite to human tumors

Unique case raises questions about misdiagnosis and treatment.

Press Release

In 2013, doctors in Colombia asked CDC to help diagnose bizarre biopsies from lung tumors and lymph nodes of a 41-year-old man who was HIV positive. The tumors looked similar to a human cancer, but initial CDC lab studies revealed the cancer-like cells were not human. That revelation kicked off a nearly three-year hunt for the cause of the man’s illness.

Researchers with IDPB, the CDC unit dedicated to investigating cases of unexplained illness and death, initially questioned whether it was an unusual cancer or an unknown infection. The growth pattern was decidedly cancer like, with too many cells crowded into small spaces and quickly multiplying. But the cells were tiny – about 10 times smaller than a normal human cancer cell. The researchers also noticed cells fusing together, which is rare for human cells.

They doggedly kept researching – performing dozens of tests – before eventually finding DNA from H. nana tapeworms in the man’s tumor in mid-2013. Unfortunately, the man died 72 hours later.

Source: Centers for Disease Control and Prevention

metastasis – cancer spread

Metastasis is the general term used to describe the spread of cancer cells from the primary tumor to surrounding tissues and to distant organs and is the primary cause of cancer morbidity and mortality

It is estimated that metastasis is responsible for about 90% of cancer deaths. This estimate has changed little in more than 50 years, says this study.

Most metastases caused by Cancer Stem Cells (CSCs).
CSCs have been shown in numerous cancer models to be involved in tumor development, cell proliferation, and metastatic dissemination, while possessing a capacity for sustained self-renewal. CSCs, which typically represent a small proportion of total cells of a given tumor, also exhibit resistance to chemotherapy and radiotherapy. Indeed, exposure to these treatments may promote “stemness” in nonstem cancer cells, which may explain why successful therapeutic reduction of tumor bulk will often fail to produce clinical improvement, says this 2016 study.

Senescent Cancer Cells are also responsible for metastasis.
Senescence is the process by which cells irreversibly stop dividing and enter a state of permanent growth arrest without undergoing cell death.

According to this 2020 Review in Trends in Cancer, a significant number of commonly used cancer interventions have been associated with the induction of cellular senescence in either tumor or non-tumor cells and tissues.

Therapy-induced senescence can cause cancer metastasis and relapse and several adverse reactions to cancer treatments.

Explore ways to kill Cancer Stem Cells and Senescent Cancer Cells

Other causes of cancer mortality

This 2019 study says:Our data support the idea that the majority of deaths from solid tumors are caused by metastases…When considering deaths from cancer without metastatic disease as an underlying cause, a few different scenarios come to mind. First, we have cases where local tumors affect vital organs, such as airways, brain, heart, and liver. Second, systemic cancer treatment and its side effects can be fatal by multiple organ failure, bleeding due to thrombocytopenia, infections, interstitial pneumonitis, and tumor lysis syndrome to name a few. Third, surgical treatment and its potential complications can also have a fatal outcome. Fourth, tragically, some patients commit suicide after a cancer diagnosis, however, a recent publication on the SEER database demonstrated this to be more frequent in patients with metastatic disease.

The scientists behind this 2018 study add the following: One of the most common questions a patient asks is: “How is this cancer going to kill me?” It is still a great mystery. Cancer rarely kills by mechanically blocking an organ – rarely does a patient die of liver failure or kidney failure because the cancer has effaced the organ.

What we know is that as patients approach a total tumor burden of approximately a kilogram of tumor (approximately one trillion cells), that this is not compatible with life.

The major syndromes associated with cancer death like cachexia or embolism appear to be mediated by cytokines released from the tumor microenvironment

Immune cells working together to defeat cancer cells.

Sources:
National Library of Medicine
National Cancer Institute
PubMed
Study
Study
National Cancer Institute

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Last updated January 2026