Final answer:
Viruses are classified based on their genetic makeup, structure, and chemical composition, with the most important aspects being the type of nucleic acid, capsid shape, and presence of an envelope.
Step-by-step explanation:
The three main criteria used in classifying viruses into families are genetic makeup, structure, and chemical composition. The genetic makeup includes the type of nucleic acid (DNA or RNA) and whether it is double-stranded (ds) or single-stranded (ss), as well as if it is linear, circular, segmented, or non-segmented. The structure relates to the capsid shape and whether a virus has an envelope. Finally, chemical composition can involve additional proteins or enzymes that are key in the viral replication process.
Viral classification systems such as the Baltimore classification scheme focus on these criteria, specifically on how messenger RNA (mRNA) is generated within the virus. This system proves to be the most commonly used method for classifying viruses today. Factors such as cell type infected, geographic distribution, and pathogenic potential are not primary criteria for classifying viruses into families, although they may be used for other categorizations or have relevance in epidemiology and treatment.