Leukemia is often lumped together, but there are actually a few different forms of the cancer—the most common in adults being one called chronic lymphocytic leukemia (CLL).

Although chronic lymphocytic leukemia makes up about a quarter of all new cases of leukemia (20,720 new cases are expected to be diagnosed this year) most people have never even heard of it. Here’s what you need to know about CLL, including how it's diagnosed, what the prognosis typically is, and how it's treated.

What is chronic lymphocytic leukemia, exactly?

In order to grasp what CLL is, it’s important to first understand leukemia in general: It's a type of cancer that starts in the blood-forming cells of the bone marrow, the American Cancer Society (ACS) explains. When one of those cells changes and becomes a leukemia cell, it grows out of control and divides to make new cells faster than it should.

Leukemia cells can build up in bone marrow and crowd out normal cells. At some point, those cells spill out into the bloodstream, increasing the number of white blood cells in the blood, the ACS says. These leukemia cells can then spread to other organs and keep cells in those areas from functioning properly as well.

Chronic lymphocytic leukemia, in particular, is a type of cancer where the bone marrow makes too many of a type of white blood cell called lymphocytes, the National Cancer Institute (NCI) says. CLL usually progresses slowly, but in some cases it can grow faster and cause a more serious disease, the ACS says.

What causes chronic lymphocytic leukemia?

Experts don’t really know. “It is not associated with typical cancer related genes,” Jennifer Woyach, M.D., a hematologist with The Ohio State University Comprehensive Cancer Center – Arthur G. James Cancer Hospital and Richard J. Solove Research Institute, tells Health. “Like many other cancers, it is probably due to the accumulation of mutations in cells over time.”

There are some risk factors for CLL, though. It’s more common in white people than those of other races, and a family history of CLL or other blood and bone marrow cancers may raise a person’s risk, Henry Chi Hang Fung, MD, FACP, vice chair in the Department of Hematology/Oncology at Fox Chase Cancer Center, tells Health. “Certain herbicides and insecticides, including Agent Orange used during the Vietnam War, have been linked to an increased risk of chronic lymphocytic leukemia,” he says. “Unfortunately, we still know very little.”

CLL usually happens at or during middle age or later in life, and it rarely happens in children or those under 40, the NCI says.

What are the symptoms of chronic lymphocytic leukemia?

CLL doesn’t usually cause any signs and can be picked up during a routine blood test before a patient has any symptoms, Dr. Woyach says. “CLL tends to start as a very slow growing disease, so people can have it for many years before knowing about it,” she adds.

However, when symptoms happen, per the NCI, they typically include:

Painless swelling of the lymph nodes in the neck, underarm, stomach, or groin

Fatigue

Feeling pain or fullness below the ribs

Fever and infection

Easy bruising or bleeding

Flat, tiny, dark red spots under the skin (caused by bleeding)

Weight loss for unknown reasons

“The diagnosis is usually established by a blood test, though sometimes a lymph node biopsy is necessary,” Dr. Fung says.

What treatments are available for chronic lymphocytic leukemia?

CLL is “highly treatable [and] all stages of CLL are treatable,” Dr. Fung says. There’s actually “no advantage to early therapy. Therefore, it does not really matter what stage the cancer is diagnosed in,” Dr. Woyach says.

Treatments for CLL “have improved dramatically” over the past decade, Dr. Woyach says, and, in most cases, patients won’t receive chemotherapy. Instead, they’ll usually undergo targeted therapy, she says, a type of treatment that uses drugs or other substances to identify and attack the cancer cells. Targeted therapy usually causes less harm to normal cells than chemotherapy or radiation therapy, the NCI says.

Immunotherapy can also be helpful, the NCI says. This is a form of treatment that uses the patient’s immune system to fight the cancer. With immunotherapy, substances made by the body or created in a lab are used to boost and direct a patient’s body’s natural defenses against cancer.

And in some cases, radiation therapy (a treatment that uses high-energy X-rays or other forms of radiation to kill cancer cells) is used, the NCI says.

Overall, most patients who are diagnosed with CLL go on to lead healthy lives. “We expect that CLL patients will live many years after diagnosis and treatment,” Dr. Woyach says.