Brain Fart -- Or Brain Tumor? Half of people are walking around with brain tumors -- that's according to singer Sheryl Crow's rep, commenting on meningioma, the benign tumor that Crow was diagnosed with last week. Ah, wrong. While meningioma is the most common brain tumor in people over the age of 35, according to a Central Brain Tumor Registry of the United States report, studies have suggested that it's more like 1 to 3 percent of the population who harbor a tumor unknowingly. Here's what you need to know about the disease. The Tumor Is Usually Harmless Meningiomas are a growth of cells on the meninges -- the layer that covers your brain, says Dr. Allen Sills, M.D., professor at Vanderbilt University. "These growing cells sit on top of the brain tissue and deform it, rather than invading it directly in most cases." The good news: The benign cells grow very slowly, and in some cases grow very little at all. In most cases, the tumors are discovered by accident, says Sills. "It's rare that someone ends up dying from this tumor -- it's one type of brain tumor that is frequently treated with great success." (Keep your brain sharp with these 6 tips .) MORE USEFUL STUFF Seek an Experienced Specialist A normally slow-growing tumor doesn't come without warning: Some tumors grow more rapidly and behave like a malignant (cancerous) tumor, Sills adds. "They can disturb brain tissue by creating increased pressure inside the head or by displacing