Why does the white blood cell (WBC) count go below normal with a viral infection?

I've read the a bacterial infection can cause high WBC count, but a viral infection can cause a low WBC count. Why does the WBC go lower with a virus, but go higher with a bacterial infection.

Well, maybe. You can actually see both high and low WBC counts with bacterial AND viral infections.

So why can the WBC count go below normal with a viral infection? First you have to try to determine WHY the WBC count is low. For example, a low white blood cell count can be the result of a prior infection - too many WBCs have been called into action and have been used up. This makes an individual more susceptible to outside infections like viruses. So when a blood test is performed, the WBC will be low, and the person will have a secondary viral infection going on.

However, there are viruses that can lower the number of a particular type of white blood cell. For example, with neutropenia, certain viruses (e.g. Hepatitis B virus) can decrease the production of neutrophils. So, you may see a below normal neutrophil count in a viral infection.

The cautionary tale is this - viral and bacterial infections can BOTH raise/lower WBC counts. Leukopenia can be seen in some viral infections (HIV) or severe bacterial infections. Leukocytosis can be seen in some viral infections (infectious mononucleosis) or bacterial infections.

Granulocytes (polymorphonuclear leucocytes): leukocytes characterised by the presence of differently staining granules in their cytoplasm when viewed under light microscopy. These granules are membrane-bound enzymes which primarily act in the digestion of endocytosed particles. There are three types of granulocytes: neutrophils, basophils, and eosinophils, which are named according to their staining properties.
Agranulocytes (mononuclear leucocytes): leukocytes characterized by the apparent absence of granules in their cytoplasm. Although the name implies a lack of granules these cells do contain non-specific azurophilic granules, which are lysosomes [2]. The cells include lymphocytes, monocytes, and macrophages.

Depending on the illness some "cytes" will either increase or decrease.

An excellent article can be found by going to http://en.wikipedia.org/wiki/White_blood...