Leukemia affects the body by disrupting the normal balance of cells in the blood and their ability to do their jobs. Reduced levels of white blood cells, which normally defend the body from infection, can lead to higher chances of catching secondary diseases. Reduced levels of platelets, the cells that cause blood to clot, lead to more frequent bruises and nosebleeds. If the cancerous cells spread to the brain, headaches and nausea can result from tissues around the brain and spinal cord becoming irritated and inflamed. Finally, painful bones can result from the cancerous cells spreading into the bone marrow.