Chains of infection: The description of the course of infection among the group of individuals. The susceptibles infected by the direct contact with the introductory cases are said to make up the ?rst generation of cases; the susceptibles infected by the  direct contact with the ?rst generation are said to make up the second generation and and this carries on. The enumeration of the number of cases in each generation is said to be an epidemic chain. Hence the sequence 1-2-1-0 denotes the chain comprising of one introductory case, the cases in ?rst two generation, one case in second generation and no cases in the later generations.