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A risk factor is anything that raises your chance of getting a disease such as cancer. Different cancers have different risk factors. Some risk factors, like smoking, weight, and sun exposure, can be controlled. Others, like your age or family history, can’t be changed.

Having a risk factor for Merkel cell carcinoma (MCC), or even many risk factors, doesn't mean that you will get it. Most people with risk factors never get MCC, while others with this disease may have few or no known risk factors.

These are some known risk factors for MCC:

Merkel cell polyomavirus (MCV) infection

The Merkel cell polyomavirus (MCV) is found in the cancer cells of about 8 out of 10 people with MCC. MCV is a common virus. Most people are infected with it at some point (often as children). But the infection doesn’t cause symptoms, and it rarely leads to MCC. Because of this, there are no recommended screening tests or treatments for MCV infection.

MCV was first discovered in 2008, and scientists are still learning about this virus. For example, it’s not clear how exactly how it might cause MCC, or why there are so few cases of MCC when infection with MCV is very common.

Ultraviolet (UV) light exposure

Exposure to ultraviolet (UV) rays is thought to be a major risk factor for most skin cancers, including MCC. UV rays damage the DNA inside skin cells. This can lead to skin cancer when this ongoing damage affects the DNA of genes that control skin cell growth.

From the sun: Sunlight is the main source of UV rays. Most MCCs start in areas of the body often exposed to the sun, such as the face, neck, and arms. People who get a lot of sun exposure are at greater risk for MCC. UV rays make up only a very small portion of the sun’s rays, but they are the main cause of the skin damage caused by the sun.

From tanning beds: Tanning beds are another source of UV rays. MCC is a rare cancer, and no studies have looked for a link between MCC and tanning bed use. But it stands to reason that more exposure to UV rays increases the risk.

From psoriasis treatments: Some patients with psoriasis (a long-lasting inflammatory skin disease) are given medicines called psoralens along with UV light treatments, this is known as PUVA treatments. This can increase the risk of MCC.

To learn more about the effects of UV rays on the skin and what you can do to protect yourself and your loved ones, see How Do I Protect Myself from Ultraviolet (UV) Rays?

Having light-colored skin

The risk of MCC is much higher for White people than for African Americans or Hispanics. Nearly all (more than 9 out of 10) MCCs are diagnosed in White people. This is probably because darker skin has a protective effect against the damaging effects of UV rays.

Being older

The risk of MCC goes up as people get older. In fact, this cancer is very rare in people under the age of 50. About 8 out of 10 people with MCC are over age 70. The increased risk may be related to skin damage caused by sun exposure over time and the fact that people’s immune systems tend to become weaker as they get older.

Being male

Men are 2-times more likely than women to develop MCC. This might be because they tend to get more sun exposure.

Having a weakened immune system

The immune system defends the body against germs such as viruses. It also seems to help the body fight cancer. People with weakened immune systems (from certain diseases or medical treatments) are more likely to develop some types of cancer, including MCC.

For example, people who get organ transplants usually are given drugs that weaken (suppress) the immune system to help keep them from rejecting the new organ. This increases their risk of developing MCC. People with autoimmune diseases (like lupus) sometimes take medicines that suppress the immune system, which might increase their risk for other diseases.

People with HIV, the virus that causes AIDS, often have weakened immune systems and are also at increased risk for MCC.

People with some types of blood cancers, such as chronic lymphocytic leukemia (CLL) or certain lymphomas, also tend to have weakened immune systems. This can be from the cancer itself, or from its treatment. People with these cancers are more likely to get MCC.

MCCs in people with weakened immune systems tend to grow faster and are more likely to be life-threatening.