context: Myelofibrosis ( MF) is a rare kind of blood cancer that starts in your marrow, a spongy tissue inside your bones that makes blood cells. The disease causes scars called fibrosis, which affects how many blood cells your body can make. MF is long-lasting and usually gets worse slowly. You may be able to live with it for years without a problem. But some people's myelofibrosis grows more quickly and causes symptoms that need to be treated. A problem with one of your genes causes you to make stem cells that don't work the way they should. These are the cells that make blood in your bone marrow. With MF they get inflamed, and scar tissue forms. About 90% of people who get this kind of cancer have a change in one of three genes: JAK2, CALR, or MPL. These genes change during your lifetime, but we don't know why. Maybe you were exposed to toxic chemicals or radiation. In most cases, you don't inherit these gene problems from your parents, and you don't pass them on to your children. These faulty genes make copies of themselves. The bad versions spread through your marrow and try to stop your body from making normal blood cells. We don't know how to prevent MF. But there's research under way to find out more about it. Most people are diagnosed around age 60. Both men and women can get it. About 18,000 people in the U. S. are living with MF. Young adults or small children can get myelofibrosis, but it's rare. Girls are affected twice as often as boys when it happens in childhood. You may get MF by itself. Or it could happen if you have another type of cancer that spreads to your marrow. Blood cancers like leukemia or myeloma could also bring it on. Long-term exposure to radiation or toxic chemicals like benzene could make you more likely to get MF. But that doesn't happen often. Blood cells: You have three types. They travel from your marrow to the rest of your body. Each has a special job to do. But if MF slows production, that can't happen. Red blood cells bring oxygen to your organs and tissues like muscles. If you have too few (your doctor will call this anemia), you might feel weak, short of breath, lightheaded, or really tired. You might have bone pain. White blood cells help you fight off infections. If you have too many, your body can't defend you from illness like it's supposed to. Platelets make your blood clot when you get a cut so you can form a scab and heal. Without enough working platelets, it may be hard for you to stop bleeding. Organs: Because your marrow has problems making blood cells, organs like your spleen, liver, or lungs may start the process instead. You could also make blood cells in your spinal cord or lymph nodes -- small glands in your groin, neck, and armpits. All that extra blood can cause organs to get too large, especially your spleen. You might feel pain or fullness in your belly if that happens. This can be serious, so you need to get it looked at right away. See your doctor regularly to check your blood for any problems. About 20% of people with MF could get acute myeloid leukemia, a kind of cancer that's harder to treat. A cancer diagnosis isn't easy. It can help to talk to other people with MF or cancer. The Leukemia and Lymphoma Society and American Cancer Society both offer online advice and local support groups.