Biological classifications are made up of abstract classes created by biologists such as species, genera, families, and orders. Members of the lower species class are also members of the classes above it and this class inclusion is responsible for the fact that genera and families, for example, have more members than species and therefore require fewer properties to meet the qualification for membership. Class membership is the logical relation that allows a link to be established between physical organisms and conceptual constructs such as classes and taxa. A philosophical approach to science is useful for analyzing the ontology and epistemology of viruses and of living organisms, and it has led to a processual interpretation of organisms and viruses in terms of continuous self-generating processes of development and evolution. The need to distinguish between the antigenicity and immunogenicity of viral proteins is emphasized because confusion between these two concepts makes it very difficult for vaccinologists to develop vaccines.