Abstract The spread of an acute contagious disease is viewed as a chain of contagion whose links are not individual cases but the smallest social groups behaving as self-contained units of an epidemic. The subsequent cases appearing in the social group after introduction of the disease are neglected unless they transmit the disease to other households. Introductory cases and their school-attendance status are employed for tracing the time and space sequences of attacks to social groups and for disclosing influencing social factors. Conceptual frameworks including geographical distance are proposed for establishing the source household when information on personal contacts with previous cases is available or when it is missing. The soundness of this approach is shown by its application to a real epidemic of variola minor, the mild form of smallpox, and is supported by findings in numerous epidemics of various contagious diseases. The study epidemic was shown to be an aggregation of rather individualized subchains and practically isolated links (households). The reality of a basic postulate of theoretical epidemiology was thus found. The number of affected households, duration of the epidemic and invasion of new areas mostly depended on elementary schools acting as diffusion agencies.