Difference Viruses Viroids Prions

Infectious disease can result of cellular organisms, such as bacteria (prokaryotes), from eukaryotes (cells like ours) or from nonliving infectious agents such as viruses, virioids and prions. Here is a summery of the different types of acellular, nonliving infectious agents.

* What Is a Cell? *

Living things are constructed of cells and can be unicellular (one cell) or multicellular (many cells).

Limits on Cell Size: Cells size is limited because cells must be able to exchange materials with their surroundings, and surface area relative to the volume decreases as size of cell increases. This limits the size of cells.

Cell Theory: The basic rules that apply to these smallest units of life state:

* All organisms are composed of one or more cells.
* Cells are the basic unit of structure and function in organisms.
* All cells come only from other cells.

* Acellular Particles: Viruses, Viroids & Prions *

Although they may seem to behave like living things, acellular particles are not alive.

Acellular particles:

* are not made of cells
* cannot reproduce on their own
* do not grow or undergo division
* do not transform energy
* lack machinery for protein synthesis
* are so small that they can only be seen with an electron microscope

* What Is a Virus? *

Viruses infect living cells and once inside, transform the cell essentially into a factory for making more viruses. These acellular particles are composed of nucleic acid (genetic material), proteins and, in some cases, lipids as well.

Viruses reproduce via four basic steps. Viral reproduction includes:

1. Delivery of the viral genetic material into a host cell
2. Commandeering of the host cell transcription and translation machinery
3. Use of the host cell’s building blocks to copy viral genomes and synthesize viral proteins
4. Viral genomes and proteins then self-assemble and exit host cells as new infectious particles.

Viruses exist in one of two states; extracellular and intracellular.

Extracellular State: Before it invades a host cell, a virus is in the ‘extracellular state’. An extracellular virus, called a virion (vie-ree-on), consists of a protein coat (capsid) surrounding nucleic acid. In addition, some viruses have phospholipid envelope surrounding the capsid.

Intracellular State: Once the virus invades a host cell it is in an ‘intracellular state.’ In this state, the capsid is removed and the virus exists as only as nucleic acid (genetic material).

* What Is a Viroid? *

Viroids are smallest known agents of infectious disease. Whereas viruses are made up of nucleic acid encapsulated in protein (capsid), viroids are uniquely characterized by the absence of a capsid. Thus far, this type of acellular particle has only been identified as an infectious agent in plants.

* What Is a Prion? *

Prions are an abnormal form of a normally harmless protein that cause various fatal neurodegenerative diseases called Transmissible Spongiform Encephalopathies. Once present in the brain, prions cause normal proteins to refold into abnormal shapes, destroying neurons and eventually causing the brain to become riddled with holes.


Bauman, R. (2005) Microbiology.
Park Talaro, K. (2008) Foundations in Microbiology.