The fitness celebrity Richard Simmons said he was treated for basal cell carcinoma. Here’s how to recognize and prevent it.

Basal cell carcinoma is the most common form of skin cancer, but it can be easy to miss, or mistake for another skin issue.

Doctors often discover the cancer during a routine skin check, said Dr. Melissa Piliang, chairman of the dermatology department at Cleveland Clinic. “A patient may not have even noticed” signs of the disease, she said.

In a Facebook post this week, the health and fitness personality Richard Simmons announced he had been treated for basal cell carcinoma. He said he first noticed a “strange looking bump” under his eye that he tried treating with Neosporin. It was only after seeing a dermatologist that Mr. Simmons was diagnosed with basal cell carcinoma.

While it can be difficult for patients to identify, basal cell carcinoma — which is estimated to affect several million people in the United States each year — is very treatable. Here’s what to know about causes, prevention and treatment.

People usually develop basal cell carcinoma after they are exposed to UV radiation through sunlight, tanning beds or sun lamps. The disease is the result of cumulative, chronic exposure, said Dr. Karen Connolly, a dermatologist at Memorial Sloan Kettering Cancer Center. Research suggests it is most common in adults over age 40.

The disease starts in basal cells, which are found in the outer layer of the skin. Another kind of skin cancer can develop in squamous cells.

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