 [ Symptoms ] how are bacterial and viral infections diagnosed [ Disease ] in general you should consult your doctor if you think you have a bacterial or viral infection exceptions include the common cold which is usually self limiting and not life threatening in some cases it is difficult to determine the origin of an infection because many ailments    including pneumonia meningitis and diarrhea    can be caused by either bacteria or viruses but your doctor usually can pinpoint the cause by listening to your medical history and conducting a physical exam if necessary he or she also can order a blood or urine test to confirm a diagnosis or a culture test of tissue to identify bacterial or viral growth occasionally a biopsy of affected tissue may be required