called "haemadsorption" or "hemadsorption". Some viruses produce localised "lesions" in cell layers called plaques, which are useful in quantitation assays and in identifying the species of virus by plaque reduction assays. Viruses growing in cell cultures are used to measure their susceptibility to validated and novel antiviral drugs. Serology Viruses are antigens that induce the production of antibodies and these antibodies can be used in laboratories to study viruses. Related viruses often react with each other's antibodies and some viruses can be named based on the antibodies they react with. The use of the antibodies which were once exclusively derived from the serum (blood fluid) of animals is called serology. Once an antibody–reaction has taken place in a test, other methods are needed to confirm this. Older methods included complement fixation tests, hemagglutination inhibition and virus neutralisation. Newer methods use enzyme immunoassays (EIA). In the years before PCR was invented immunofluorescence was used to quickly confirm viral infections. It is an infectivity assay that is virus species specific because antibodies are used. The antibodies are tagged with a dye that is luminescencent and when using an optical microscope with a modified light source, infected cells glow in the dark. Polymerase chain reaction (PCR) and other nucleic acid detection methods PCR is a mainstay method for detecting viruses in all species including plants and animals. It works by detecting traces of virus specific RNA or DNA. It is very sensitive and specific, but can be easily compromised by contamination. Most of the tests used in veterinary virology and medical virology are based on PCR or similar methods such as transcription mediated amplification. When a novel virus emerges, such as the covid coronavirus, a specific test can be devised quickly so long as the viral genome has been sequenced and unique regions of the viral DNA or RNA identified. The invention of microfluidic tests as allowed for most of these tests to be automated, Despite its specificity and sensitivity, PCR has a disadvantage in that it does not differentiate infectious and non-infectious viruses and "tests of cure" have to be delayed for up to 21 days to allow for residual viral nucleic acid to clear from the site of the infection. Diagnostic tests In laboratories many of the diagnostic test for detecting viruses are nucleic acid amplification methods such as PCR. Some tests detect the viruses or their components as these include