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Classes of infectious agents and why we care

Why do we care what kind of infectious agent is causing a disease?

Why do we care what kind of infectious agent is causing a disease?

We care because in order to understand the pathology of an infectious disease, in order to know how to treat it, you must understand the infectious agent behind it.

There are six main classes of infectious agents:

  • Prions
  • Viruses
  • Bacteria
  • Fungi
  • Protozoa
  • Helminths

Each of these is described in more detail below. Some infectious agents are organisms, which are living things that are unicellular (made up of one cell) or multicellular (made up of multiple cells). The basic structure of the cell differs across classes of organisms. Cells can either be eukaryotic or prokaryotic.

Individual cells contain organelles (tiny cellular structures) that perform unique functions. Eukaryotic cells are much more complex than prokaryotic cells as the former contain many different types of membrane-bound organelles whereas in prokaryotes (which are much smaller and simpler cells), they have limited intracellular structures. Notice this difference in the two structures in the image below.


Prions are misfolded proteins, which can cause properly folded proteins to convert to the misfolded prion form. A chain reaction occurs as the new prions then go on to convert more proteins. Not only are these new prions no longer able to perform their normal function, they accumulate within infected cells, causing tissue damage and cell death. As they reside in host cells, they are difficult to detect and target, and very little is known about them compared with other infectious agents.


Like prions, viruses are non-living infectious agents that reside inside host cells. They consist of either a DNA or an RNA genome enclosed within a protein coat called a capsid. Some viruses are enveloped, meaning that their capsids are covered with a lipid layer called an envelope. Viruses lack the means to replicate themselves; instead they force the host cell to do it for them. This replication process often disrupts or kills the host cell. Some viruses have the ability to integrate themselves into the host’s DNA only to re-emerge later in the host’s life (for example a retrovirus).


Bacteria are unicellular prokaryotic organisms that consist of a very simple prokaryotic cell consisting of a cell membrane surrounded by a cell wall, and containing a simple circular chromosome. Bacteria are broadly divided into three types according to the structure of their cell wall: Gram positive, Gram negative and acid-fast, which refers to the way these bacteria react to particular stains used in the laboratory. Bacteria can live and replicate outside host cells, which makes them much easier to detect and target compared to prions and viruses. Some bacterial species can form endospores or cysts, dormant (inactive) forms to survive extremely harsh conditions for long periods.


Fungi are eukaryotic organisms that can exist in and switch between two forms: a unicellular form (yeast) and a multicellular form (filamentous) which is often referred to as mould. They are usually opportunistic pathogens, only causing disease in compromised hosts. Some fungi can reproduce via the creation of spores – a resilient microscopic particle which can survive in harsh conditions for long periods of time.


Protozoa are a considerably diverse group of unicellular eukaryotic organisms. Species differ so significantly from each other that antiprotozoals are commonly only effective against one species. Some protozoa life cycles alternate between a fertile stage and a dormant cyst.


Helminths are parasitic worms, and are multicellular and eukaryotic. Unlike other infectious agents, Helminths produce eggs, each one capable of producing thousands of new worms. These eggs have tough shells that protect them from a wide range of environmental conditions for long periods of time.

Treatment of infectious disease

In order to treat an infectious disease, it is essential to know which agent is the cause. Antimicrobials work in very specific ways, and thus may only be effective on specific infectious agents. For example, the antibiotic penicillin works by disrupting the development of peptidoglycan cell walls, and thus wouldn’t be effective against a fungal disease as fungal cell walls are not comprised of peptidogylcan. Fungal cells and protozoa are biologically similar to animal cells (ie they are all eukaryotic). This creates difficulties in developing drugs that target the infectious agent without harming animal cells.

Your Task

We’ve likely all experienced infection such as colds, flus and various stomach bugs. What other kinds of infections have you or anyone you know experienced? What treatment was used and was it effective? Share your responses with the others in the course.


Lee, G. & Bishop, P. (2016). Microbiology and infection control for health professionals. Melbourne. Pearson Australia.

© Griffith University
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