Causes of low white blood cells (leukopenia) include: infections (both viral and HIV), autoimune disorders (lupus), certain medications, radiation therapy or bone marrow disease. You've experienced infection of the urinary tract. During most infections (especially the bacterial ones), the white blood cell count goes up. After the infection is overcome, it comes to its normal levels. If the white blood cell levels continues to fall down you may need to take into consideration the aforementioned possibilities and request diagnostic examinations from your general practitioner.