The most common reason for a low white blood cell count (leukopenia), particularly neutropenia (a decrease in neutrophils, a key type of white blood cell), is infection. Viral infections are especially frequent causes, as they can temporarily affect bone marrow function and reduce white blood cell production. Severe infections may also cause the body to use up white blood cells faster than they can be produced

. Other common causes include:

• Cancer and cancer treatments: Leukemia and other cancers affecting the bone marrow, as well as chemotherapy and radiation therapy, can impair white blood cell production

• Autoimmune diseases: Conditions like rheumatoid arthritis and lupus can lead the immune system to destroy white blood cells

• Nutritional deficiencies: Lack of vitamin B12, folate, copper, and other nutrients necessary for white blood cell production can cause low counts

• Medications: Besides chemotherapy, some antibiotics and other drugs can cause leukopenia as a side effect

• HIV infection: This virus attacks immune cells and leads to a low white blood cell count

In summary, while many factors can cause low white blood cell counts, the most common reason in adults is viral infection, which temporarily suppresses bone marrow production or increases white blood cell usage