The Usual Suspects: Pathogens That Cause Infectious Disease

Part I of the Signal to Noise Infectious Disease Primer

We are constantly faced with news reports about infectious disease, but the sensationalism surrounding these diseases often drowns out important information. This article is the first of a three-part series exploring the basics of infectious disease, the factors contributing to disease incidences, and the steps scientists are taking to understand the origins of emerging infectious diseases. 


This article is part of our Infectious Disease theme! Click here to read more about infectious disease.


Infectious diseases are those that are readily spread between hosts, and are caused by organisms called pathogens. Pathogens may infect a person and can cause disease, and spread between individuals through direct or indirect contact. This article will focus on the four groups of microorganisms (organisms that are a million times smaller than us)  responsible for causing disease: viruses, bacteria, fungi and parasites [1], [2].    

Most viruses have a protective membrane coat, surrounding a protein capsid that contains DNA or RNA. “HI-virion-structure” by Thomas Splettstoesser licensed under CC BY 4.0. / Labels added to original by Nisar Farhat 

In recent years, viruses have made headlines multiple times. In 2009 the World Health Organization (WHO) declared the H1N1 influenza outbreak an international public health emergency [3]. This declaration represents the highest level of alert and has since been issued three more times: in 2014 as poliomyelitis re-emerged in Pakistan and Syria [4], again in 2014 during the Ebola virus outbreak in West Africa [5], and during the current Zika virus epidemic [6]. Despite their ferocity, viruses are the simplest form of infectious pathogens. Generally, a virus is comprised of a hard protein shell, called a caspid, that contains a strand of DNA or RNA, and an outer protective membrane coat, which allows the virus to enter the host cell.


How do these simple organisms cause disease? In general, a virus infects its host and hijacks host cells to replicate. Through this hijacking, the host’s body stops functioning properly, producing the symptoms of the disease. To date, the best preventative measure against the spread of viruses is vaccination. Vaccines produce immunity in the vaccinated individual against the pathogen. This works well as a preventative measure but remains a challenge for newly emerging pathogens such as Zika virus which have no established vaccine. Once infection occurs, there are antiviral therapies available, though they are limited. In most cases, antiviral drugs are virus-specific and prevent specific steps in the virus lifecycle such as cell entry or DNA replication [7].


The other types of pathogenic microorganisms on our list are often more complex than viruses. Bacteria, for example, are prokaryotic single-celled organisms. All prokaryotes, including bacteria, lack a membrane-bound nucleus and membrane-bound organelles, and store their DNA in the form of a tangled bundle called a nucleoid. To reproduce, a parent bacterium makes copies of each individual item in the cell, including DNA, and then simply divides to make an identical copy of itself. Everything is covered in bacteria; in fact, bacteria constitute a larger biomass on this planet than all plants and animals combined. Furthermore, not all bacteria are harmful: your gut and skin are covered in bacteria which contribute to your health [8]. Recent research into gut bacteria has yielded intriguing results with implications for numerous diseases, including diabetes and inflammatory bowel disease [9].


Bacteria cause disease by invading the host, but unlike viruses, which tend to target specific tissues in the host, bacteria often spread indiscriminately. Symptoms often include fever, pain, and pus where bacteria settle. Bacteria themselves are not always the cause of disease; in some cases it is the toxin the bacterium produces that is dangerous to the host’s health. The types of toxins bacteria produce are broken into two categories: endotoxins and exotoxins. Endotoxins are attached to the cell wall surrounding the bacteria, making the bacterium itself toxic to the host. Exotoxins are often proteins produced by bacteria; these cause harm by diffusing throughout the body and harming the infecting individual [10]. The main course of action against bacteria is antibiotic treatment. These drugs break down bacterial cell walls or membranes, as well as inhibit the function of crucial bacterial proteins required for DNA replication and energy. In some cases, vaccines are also used against bacteria, such as the vaccine protecting you from Clostridium tetani that causes tetanus [11].

A eukaryotic cell (left) is more compartmentalized with specific structures and membrane bound organelles. In contrast, prokaryotic cells (right) lack a membrane bound nucleus and organelles. “Celltypes” by SVG and NCBI, part of the NIH, and is part of the public domain.

The final two members on our list are fungi and parasites, which are generally eukaryotic organisms, making them the most biologically complex than viruses and bacteria. Unlike bacteria, which are prokaryotes, the cells of eukaryotes contain membrane-bound nuclei and organelles[2] , giving the cell the ability to compartmentalize specific cellular tasks.


Besides being biologically more complex than viruses and bacteria, these types of infectious pathogens also cause disease through different mechanisms. Fungi in large part affect people with a weakened immune system - many fungal infections are opportunistic infections. Common skin infections or vaginal yeast infections are not generally life-threatening to a healthy person. In contrast, lung infections like Valley fever can be more serious and may be the result of the person being exposed to fungal spores [12]. Fungal infections can be treated with creams for skin infections or antifungal medications.


Parasites are different still; these organisms scavenge food at the expense of their host [13]. Parasites can also be carriers of other infectious disease. Organisms that transmit diseases between humans or from animals to humans, but are themselves not affected, are called vectors [14]. An example of a parasitic vector is the deer tick that carries Lyme disease caused by the bacteria called Borrelia burgdorferi [15]. Unfortunately, people in the tropics, subtropics, and sub-Saharan Africa carry the largest disease burden of parasitic disease. Some parasitic infections such as malaria can be treated with drugs,  while others require physical or surgical medical intervention such as the removal of some parasitic worms. However, many parasitic diseases remain untreated and do not have an immediate treatment options. Many of these belong to a group of disease that has not received adequate attention from the public health community called Neglected Tropical Disease (NTDs), which we will explore in more detail in Part II of this series [13].


This was a basic overview of the four types of microorganism which cause infectious diseases. In Part II, we dive into how disease are carried by vectors and spread, while Part III will document how scientists gather information about and approach emerging infectious disease.


Nisar Farhat
Co-Founder and CFO, Signal to Noise Magazine
PhD Candidate, Molecular and Medical Pharmacology, UCLA



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2.     Keesing, F., “Impacts of biodiversity on the emergence and transmission of infectious diseases. Nature 468, 647-652, (2010).

3.     World Health Organization. “Pandemic (H1N1) 2009”. WHO. 2016.

4.     World Health Organization. “Poliomyelitis”. WHO. 2016.

5.     World Health Organization. “Ebola outbreak 2014-2015”. WHO. 2016.

6.     World Health Organization. “Zika virus disease”. WHO. 2016. Clercq, E.D. “Recent highlights in the development of new antiviral drugs”. Current Opinions in Microbiology 8, 552-560, (2005).

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10.  Montecucco, C., Schiavo, G. “Mechanism of action of tetanus and botulinum neurotoxins”. TOC 13, 1-8, (1994).

11.  Center of Disease Control. “Fungal Diseases”. CDC. 2016

12.  Center of Disease Control. “Parasites”. CDC. 2016

13.  World Health Organization. “Vector-borne Disease”. WHO. 2016.

14.  Steere, A.C., Coburn, J., Glickstein, L. “The emergence of Lyme disease.” J Clin Invest 113, 1093-1101, (2004).