What distinguishes viruses from other infectious organisms?

Viruses are unique among infectious organisms due to their reliance on a host to reproduce. Unlike bacteria or protozoa, which can grow and multiply independently in various environments, viruses lack the cellular machinery necessary for self-replication. Instead, they must invade a host cell and hijack the host's biological processes to produce more viral particles. This dependency on a host for reproduction is a fundamental characteristic that distinguishes viruses from other types of pathogens.

The other options do not accurately describe viruses. For example, while some infectious organisms might infect multiple species, this characteristic is not exclusive to viruses, nor does it define them. Additionally, viruses are not composed of living cells; they are fundamentally different in structure, consisting of genetic material encased in a protein coat, and in some cases, a lipid membrane. Their inability to reproduce independently reinforces their classification as non-living entities when outside of a host system.