(I) Viruses are more closely related to chemical matter than to a living organism. Viruses are acellular, inert outside host cells, and can be crystallized, so they are often considered at the border of living and non-living.

(II) Viruses and bacteria are the only infectious agents found in plants and animals.

Other infectious agents also exist, such as fungi, protozoa, helminths, prions, and viroids.

(III) All viruses are obligate parasites of cells, the majority do not cause disease.

Viruses require living cells to reproduce, and many infect hosts without causing obvious disease.

(IV) Viruses cannot be observed using a light microscope.

Most viruses are below the resolving power of a light microscope and require an electron microscope.

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