Malignancy (infiltration in leukemia, myeloproliferative disorders or bony metastases)
B12 or folate deficiency
Aplastic anemia
Infection
Sepsis
Acute viral infection (e.g. URI, influenza, HIV)
HIV/AIDS
Typhoid, dengue, malaria, rickettsia, lyme
Autoimmune disorders (e.g. SLE, Crohn's, RA)
Drugs/tox
Chemotherapeutics (antineoplastics, hydroxyurea, methotrexate)
Radiation exposure
Clozapine, chlorpromazine
Phenytoin
Sulfonamides, penicillin, cephalosporins, antiretrovirals
Cimetidine
Hypersplenism/splenic sequestration
Miscellaneous
Congenital, cyclic, "chronic benign neutropenia"
Sepsis
Acute viral infection (e.g. URI, influenza, HIV)
HIV/AIDS
Typhoid, dengue, malaria, rickettsia, lyme
Chemotherapeutics (antineoplastics, hydroxyurea, methotrexate)
Radiation exposure
Clozapine, chlorpromazine
Phenytoin
Sulfonamides, penicillin, cephalosporins, antiretrovirals
Cimetidine
Congenital, cyclic, "chronic benign neutropenia"
Leukopenia
Malignancy 
Pancytopenia
B12 or folate deficiency
Thrombocytopenia
Aplastic anemia
Thrombocytosis
Infection