What cause your whiteblood count to be low is bad number to have 2.8?

Possibly viral. Lots of causes to have a low white cell count, however, probably one of the most common and benign ones would be a viral infection such as the common cold. Shortly after having had such an infection the white cell count will come back to normal. Other diagnoses for this finding would likely result in additional things being abnormal but that doesn't sound like it's the case. Is your doc concerned?