Bioassays are the gold standard for detecting the infectivity of tissues.
They are therefore used in transmission experiments to provide a model for assessing the likelihood that infective tissues could act as agents for transmitting disease. The bioassays method is able to defect infectivity directly, rather than by relying on a correlation with the presence of modified PrP as defected by alternative methods, usually immunological or histological. This tissue to be tested is either fed to, or injected into the experimental animal, which is then observed for signs of the disease. Based on a combination of observations of incubation period and brain pathology, as well as other factors, assumptions can be made about the infectivity of tissue and type of disease. This is called strain typing.