Genetic material exchange between viruses and their hosts Figure 1A shows two over-simplified scenarios in support of a genetic exchange from the virus to the host genome and in the reverse direction, from the host to the viral genome. In the first scenario, a viral sequence is detected in the host (e.g., human) but not in the rest of the phylogenetic branch. The following scenario accounts for viral sequences acquired from the host ( Figure 1A , right). Under this scenario, the viral gene sequence is identified in a broad group of organisms that belong to a phylogenetic tree that includes the host (human). Therefore, the sequence in the virus is most likely a reflection of a hijacking event, according to an argument of maximum parsimony.