Why in sudden outbreaks, it may be better to test for disease antigens rather than for antibodies?

In the case, when the outbreak is sudden, there is a need to test for antigen, as the body has not generated any kind of antibodies yet to defend against the disease. It is thus better to test for the antigens.  

In epidemiology, an outbreak refers to the sudden enhancement in the happenings of the disorder in a specific place and time. The outbreaks involve epidemics, which is usually applied for infectious diseases, as well as ailments with an environmental origin, like foodborne or water disease.  

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