Viruses are diagnostically important because they show why APS does not reduce life to replication, evolution, or structural organisation.
A virus outside a host is organised but not actively viability-oriented. It does not regulate, metabolise, or sustain the conditions of its own persistence. Inside a host, viral activity becomes biologically consequential, but only by exploiting the host’s metabolism, regulation, and organisational closure.
For APS, viruses are therefore best understood as organisational parasites. They participate in biological systems, perturb living organisation, and shape evolutionary and ecological dynamics, but they do not constitute autonomous viability-oriented systems in their own right.
This is why viruses sit at the boundary of life without dissolving that boundary.