Diagnosis

Common viral infections may be diagnosed based on symptoms. For infections that occur in epidemics (such as influenza), the presence of other similar cases may help doctors identify a particular infection. For other infections, blood tests and cultures (growing microorganisms in the laboratory from samples of blood, body fluid, or other material taken from an infected area) may be done. Blood may be tested for antibodies to viruses or for antigens (proteins on or in viruses that trigger the body's defenses). Polymerase chain reaction (PCR) techniques may be used to make many copies of the viral genetic material, enabling doctors to rapidly and accurately identify the virus. Tests are sometimes done quickly-for instance, when the infection is a serious threat to public health or when symptoms are severe. A sample of blood or other tissues is sometimes examined with an electron microscope, which provides high magnification with clear resolution.