Anemia can be caused by several different things. Either active bleeding, when one looses blood, resulting in low hemoglobin or if your body lacks the ability to manufacture red blood cells, as is the case with bone cancer. Red blood cells are essential, because they carry the oxygen in the blood. One who is severely anemic, is often fatigued, short of breath, pale, inattentive and may be confused. The sooner it is taken care of he better. This is not something to ignore as it can, over time, and with increased severity, lead to cardiac arrest, as the heart becomes starved for oxygen.