evidence: All viruses are ultimately parasites. They require a host for replication. They cannot generate their own energy like cells can. [...] Viruses that have been in a host for a long time are less likely to cause disease. For example, HIV jumped into humans from wild primates, in whose bodies it wasn’t causing any disease. Every virus-host relationship is different. In most cases, viruses do not cause any disease, and many are beneficial. For example, in mice a herpes virus prevents infection from the plague bacteria.