Why does pneumonia decrease hemoglobin levels?

Good question: No good answer. In the hospital, a drop in hemoglobin could be due to multiple blood draws, and or administration of intravenous fluids. Infections can suppress the bone marrow that produces red blood cells, older blood cells die earlier.

Does not usually: Pneumonia does not typically decrease the hemoglobin levels. Patients, critically ill with severe pneumonia may have severe inflammation throughout the body ( called the systemic inflammatory response syndrome) these patients may need life support. This group may have low hemoglobin levels because the red blood cells live for a shorter time and fewer are made. This is a consequence of the "sirs".