What microorganism is specifically described as an invader of plants and animals?

The correct answer is viruses, as they are unique pathogens that often invade both plant and animal cells to replicate. Viruses are made up of genetic material (DNA or RNA) encased in a protein coat and cannot reproduce on their own; they must infect a host cell to replicate and proliferate. This characteristic makes them distinct among microorganisms.

While fungi, bacteria, and protozoa can also infect plants and animals, their modes of infection and structures differ significantly. Fungi are typically decomposers and can be pathogenic, but they do not fit the model of virus-like invasive behavior. Bacteria are single-celled organisms that can cause diseases in both plants and animals but are fundamentally living entities that can reproduce independently. Protozoa are also single-celled organisms but primarily affect animals and humans, with some affecting plants in specific circumstances. Thus, viruses are specifically defined by their dependency on host cells for invasion and replication, making them the unique invaders described in the question.