A high monocyte count may be caused by tuberculosis, leukemia, a parasite infection, chronic inflammatory disease or a viral infection, states MedlinePlus. Types of viral infections include mumps, measles or infectious mononucleosis. High numbers of white blood cells are caused by any inflammation, blood disease or immune response.
Normal monocyte levels are between 2 and 8 percent, explains MedlinePlus. When one type of white blood cell has a higher-than-normal increase, this causes other types of white blood cells to have a lower-than-normal level. Infections of any type or acute stress increase the number of white blood cells. There are five different types of white blood cells in the body, which are also referred to as leukocytes. The different types of leukocytes are neutrophils, lymphocytes, basophils, monocytes and eosinophils.
The test to measure the numbers of white blood cells is called a blood differential, according to MedlinePlus. It also reveals if a person has cells that are not fully developed or abnormal in some way. A blood differential is done to test for conditions such as anemia, infections and leukemia. People who are being treated for one of these conditions may have a blood differential ordered to make sure their prescribed treatment is working.