Final answer:
The description that protozoan pathogens have the most complex life cycles better applies to them than to viral pathogens. Protozoa are diverse, unicellular eukaryotes with nutritional roles in ecosystems, whereas viruses consist of simple structures with nucleic acids and protein, relying entirely on host cells for replication.
Step-by-step explanation:
Among the descriptions listed, the one that better applies to protozoan rather than viral pathogens is that they have the most complex life cycles. Protozoan pathogens are unicellular eukaryotic parasites with various mechanisms of pathogenicity including adhesins, toxins, and antigenic variation. Unlike viruses, which are acellular entities consisting simply of nucleic acid and a protein coat, protozoa are a diverse group of organisms. Some are free-living while others are parasitic, and they can have complex life cycles involving different forms and stages, including cysts and trophozoites in their life cycle. Moreover, protozoa are part of the backbone of many food webs, contributing nutrients to other organisms and thus play a significant role in the ecosystem.
In contrast, viruses such as those causing polio and smallpox, are much simpler structures, merely consisting of nucleic acids (DNA or RNA) enclosed in a protein shell, sometimes with a lipid envelope derived from the host cell's membrane. They have a fairly straightforward life cycle involving attachment, penetration, uncoating, biosynthesis, maturation, and release, all intracellularly, with no metabolic activity or growth outside of a host cell.