electron microscopy and enzyme-immunoassays. The so-called "home" or "self"-testing gadgets are usually lateral flow tests, which detect the virus using a tagged monoclonal antibody. These are also used in agriculture, food and environmental sciences. Quantitation and viral loads Counting viruses (quantitation) has always had an important role in virology and has become central to the control of some infections of humans where the viral load is measured. There are two basic methods: those that count the fully infective virus particles, which are called infectivity assays, and those that count all the particles including the defective ones. Infectivity assays Infectivity assays measure the amount (concentration) of infective viruses in a sample of known volume. For host cells, plants or cultures of bacterial or animal cells are used. Laboratory animals such as mice have also been used particularly in veterinary virology. These are assays are either quantitative where the results are on a continuous scale or quantal, where an event either occurs or it does not. Quantitative assays give absolute values and quantal assays give a statistical probability such as the volume of the test sample needed to ensure 50% of the hosts cells, plants or animals are infected. This is called the median infectious dose or ID 50. Infective bacteriophages can be counted by seeding them onto "lawns" of bacteria in culture dishes. When at low concentrations, the viruses form holes in the lawn that can be counted. The number of viruses is then expressed as plaque forming units. For the bacteriophages that reproduce in bacteria that cannot be grown in cultures, viral load assays are used. The focus forming assay (FFA) is a variation of the plaque assay, but instead of relying on cell lysis in order to detect plaque formation, the FFA employs immunostaining techniques using fluorescently labeled antibodies specific for a viral antigen to detect infected host cells and infectious virus particles before an actual plaque is formed. The FFA is particularly useful for quantifying classes of viruses that do not lyse the cell membranes, as these viruses would not be amenable to the plaque assay. Like the plaque assay, host cell monolayers are infected with various dilutions of the virus sample and allowed to incubate for a relatively brief incubation period (e.g., 24–72 hours) under a semisolid overlay medium that restricts the spread of infectious virus, creating localized clusters (foci) of infected cells. Plates are subsequently probed