Rarely a cancer sign. Leukocytosis is an abnormally high number of white blood cells in the blood. Both viral and bacterial infections can sometimes cause high numbers of white blood cells (wbc's) or low numbers of wbc's (leukopenia). Unusual leukocytosis, not caused by an infection, can be due to a bone marrow problem such as an early stage of leukemia (cancer of the blood cells). A doctor can further evaluate.