From index case to branching contacts
Contact tracing maps transmission as a tree. At the root sits the index case, the first identified infection. Edges run out to primary contacts who were exposed directly, and from any infected primary contact to their own secondary contacts. Following these branches lets investigators find people who may be incubating disease before they spread it further.
The structure is a directed graph of who infected whom, reconstructed from interviews and exposure windows. Gaps appear when contacts cannot be reached or when transmission happened in untraceable settings.
Why the branching factor reflects transmission
The average number of new infections each case produces mirrors the effective reproduction number. When that figure is above 1, the tree widens at each generation and the outbreak grows; when it is below 1, branches die out and the outbreak shrinks. Effective tracing aims to cut branches faster than they form.
This is general educational information, not medical advice. Decisions about isolation, quarantine, and testing follow public health guidance from qualified authorities.