Leukopenia is a decrease in the number of white blood cells circulating within blood. Common causes include:

• typhoid fever
• viral infection
• rickettsial infection
• chronic or fulminant infection
• after measures designed to obliterate leukocyte population e.g. chemotherapy or radiotherapy

It can be subdivided according to the white cell population that is (most) affected:

• neutropenia
• lymphopenia
• eosinopenia
• monocytopenia