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's 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.
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