Fig. 1.

The inferred rooted phylogenetic tree (the root defined using a reference strain not shown on the figure) from a hypothetical known transmission chain. The branch nodes correspond to the coalescent events of the different viral lineages; the arrows show the hypothesized time points when the transmissions happened. Even though it might seem that there was a short time between C ≥ B and B ≥ A transmissions (short genetic distance between the branching and the points of transmission showed by the arrows), in reality it might have been years in between the two events.