Hi ,
this is what I got from medicine literature.
If the number of leukocytes is strongly degraded, then this is in most cases a reference to a virus infection. Also diseases such as malaria or typhoid fever can be the cause of a decreased number of leukocytes. Likewise a damage of the marrow can accompany, e.g. in consequence of Roentgen irradiation or a treatment (cancer medicines), with a reduction of the number of leukocytes....