White blood cells are made in bone marrow — the spongy tissue inside some of the larger bones. Conditions that affect the bone marrow are the usual causes of low white blood cell count. Some of these conditions are present at birth, also known as congenital.

Causes of a low white blood cell count include:

• Aplastic anemia
• Chemotherapy
• Radiation therapy
• Epstein-Barr virus infection.
• Hepatitis A
• Hepatitis B
• HIV/AIDS
• Infections
• Leukemia
• Lupus
• Rheumatoid arthritis
• Malaria
• Malnutrition and lack of certain vitamins
• Medications, such as antibiotics
• Sarcoidosis
• Sepsis (an overwhelming bloodstream infection)
• Tuberculosis

A low white blood cell count is a decrease in the cells in the blood that fight disease.

What's low in white blood cell count varies from one lab to another. This is because each lab sets its own reference range based on the people it serves. In general, for adults, a count lower than 3,500 white blood cells per microliter of blood is considered low. For children, an expected count depends on the age.

It's possible for some people to have white blood cell counts that are lower than what's usually expected and still be healthy. For instance, Black people tend to have lower counts than white people do.

A test a health care provider orders to diagnose a condition can reveal a low white blood cell count. A low white blood cell count is rarely found by chance.

Talk to your care provider about what your results mean. A low white blood cell count plus results from other tests might show the cause of your illness. Or you might need other tests for more information about your condition.

A very low white blood cell count over time means you can get infections easily. Ask your care provider about ways to not catch diseases that are passed from one person to the next. Wash your hands regularly and well. Consider wearing a face mask and stay away from anyone with a cold or other illness.