A low white blood cell count is mainly caused by viral infections that temporarily affect the bone marrow's function. Certain diseases, which include impaired bone marrow activity, are present at birth (congenital). Cancer or other disorders that cause bone marrow destruction. Medications that destroy healthy cells or inhibit their growth. A low neutrophil count occurs when there are too few neutrophils in the blood. This can be due to loss of blood cells through trauma, illness, or infection. It can also be a sign of cancer or another disorder that affects the bone marrow. Neutropenia can also occur as a side effect of chemotherapy for cancer treatment or radiation therapy for chronic conditions such as multiple sclerosis. Low numbers of red blood cells or platelets may indicate an underlying medical problem. These could be signs of kidney disease, alcohol abuse, or blood loss due to injury or surgery. They can also be side effects of certain medications or problems with blood transfusions. Medical professionals will need to diagnose low white blood cell counts and neutrophils by reviewing your history and physical examination findings together with results from laboratory tests. They will also consider your lifestyle factors such as smoking, drinking, drug use, and obesity; as well as your family history of health issues. Your doctor may suggest tests to find out what is causing your low white blood cell count.