Select the description that would represent a pathogen that would be considered a virus

A. Genetic material that is surrounded by a protein coat and uses a host cell to reproduce.

B. A single-celled organism that infects animals and plants.

C. An organism that feeds off others in order to gain its nutrients.

D. A single-celled organism that can be a parasite.

Diagram showing the structure of the influenza virus
Diagram showing the structure of the influenza virus

The correct answer is A. Genetic material that is surrounded by a protein coat and uses a host cell to reproduce.

A pathogen is an infectious agent that can cause disease. This can include bacteria, viruses, fungi, parasites and even prions (infectious proteins). While bacteria, fungi, and parasites consist of cells, a virus does not. In fact, a virus consists of genetic material that is surrounded by a coat of proteins and can only multiply inside a living cell.

This is different from bacteria, which can often reproduce outside of a living cell. It is important to realize that viruses can be just as deadly as bacteria when they cause illness and may even be harder to kill.

One reason for this is that they often change the proteins on the outer coat and thus are able to avoid the immune system. This is the reason why a new flu vaccine has to be manufactured every year; it is because the flu virus changes the proteins on the outer coat each year.

Single-celled (unicellular) pathogenic organisms include bacteria, some fungi, and some protozoan parasites. Parasites are organisms that feed off another organism and can include protozoans such as the malaria parasite, but also multicellular organisms such as roundworms and tapeworms.

There are pathogenic fungi that can infect people, but this is most often the case when people have a poor immune system.


Viruses infect all forms of living organisms and have been responsible for much human misery and death over the years. In fact, influenza pandemics alone have killed millions of people worldwide.

Some viral illnesses have been largely eradicated by mass global vaccinations. For instance, smallpox was eradicated after many years of extensive vaccination. The structure of viral particles is very different from that of bacteria and single-celled organisms.


The virus contains a core of genetic material that is known as a nucleocapsid. This region is then surrounded by a protein coat and sometimes also an envelope. The shape and size of the genetic material, protein coats and presence and type of envelope are important for the classification of these particles.

The nucleic acids of the genetic material can and does vary from one virus to another. In fact, some viruses contain RNA instead of DNA, while others contain DNA. For instance, influenza virus, HIV and West Nile Virus all have RNA as their nucleic acid.  DNA is the nucleic acid that is present in herpes and adenoviruses.

The protein coat is often called a capsid and it consists of segments known as capsomeres. The viral shape is largely a result of how these capsomeres are arranged with some forming a polyhedral shape while others may have a helical shape.

In some cases, an envelope may also be present surrounding the protein coat. This envelope consists of lipoproteins which are proteins linked to fats.

Viruses are difficult to control especially since some species change the proteins on the coat. In influenza, for instance, the proteins are hemagglutinin and neuraminidase. These determine the strain of the virus, and unfortunately, they often change.

Your immune system is able to attack the virus particles by recognizing these proteins so when they change it becomes a problem. This is why a new flu vaccine has to be created every year to keep up with the changing strains of flu.


A virus has to enter into and infect a host cell in order to reproduce itself. The basic steps involve attachment to the cell membrane of the host cell, after which the viral genetic material enters or is injected into the host cell. Once inside the cell, many copies of viral particles are made which when released usually kill the cell.


Bacteria are single-celled living organisms that contain a cytoplasm with ribosomes and genetic material in the form of DNA. The DNA is found in a specific region of the cell known as the nucleoid, although there is no membrane surrounding the area.

A cell membrane and cell wall are present surrounding the cellular contents. The cell wall can be quite variable in its components. In addition, some species such as Streptococcus pneumonia also produces a capsule surrounding the cell wall. The capsule of this species is the reason for it being so pathogenic and is why it can cause pneumonia in people.

In fact, there are many pathogenic bacterial species which cause illness and even death in people. Most cases of food poisoning, for instance, are caused by species such as Salmonella, Listeria, and Escherichia coli.

Antibiotic resistance has also become a problem with some species of Staphylococcus becoming resistant to many antibiotics.

With a few exceptions (Rickettsia), most species of bacteria do not have to enter our cells to reproduce but can do so in the tissue fluids. They can live and reproduce on surfaces outside of the human body which is why we can have films known as biofilms developing on surfaces that are hard to get rid of.


Fungi are eukaryotes which can be pathogenic, but this occurs most often in people who have a compromised or weakened immune system. Fungal cells are eukaryotic cells which have a complex cell structure consisting of membrane-bound organelles, and a cell wall made of chitin.

Infections are more commonly on the outside surface of the body, but internal infections can occur in people with a compromised immune system. For instance, Cryptococcus can cause meningitis in infants and Aspergillosis can cause lung infections in people.

Protist parasites

The protists are also single-celled organisms which can sometimes be parasites in humans. Such parasites include the organism that causes malaria and amoebic dysentery. These organisms can be deadly and in fact, malaria alone kills millions of people every year.

These organisms are eukaryotic cells that do not have a cell wall. The internal structure of the cell is complex and some can form resistant stages to survive unfavorable conditions.

The life cycles are also commonly complex and the pathogenic forms often have life cycles involving more than one host species. For instance, the Plasmodium parasite that causes malaria is carried in the mosquito and is transported to humans when a mosquito bites someone.

Some protistans are very deadly, for instance, Naegleria fowleri, which causes a fatal meningitis of the brain. This organism has a 95% fatality rate and is sometimes found in warm standing waters such as farm dams. People get infected when they accidentally take in the water.

Other parasites

There are also multicellular animals that are pathogenic and parasites of people and other animals. Hookworms, roundworms, and tapeworms are all examples of such organisms that live inside people and take their nutrients from our bodies.


  1. S Baron (1996). Medical Microbiology, 4th edition. Galveston: USA, University of Texas Medical Branch at Galveston.
  2. RR Krug, RR Wagner (2018). Virus. Retrieved from Encyclopedia Britannica.
  3. RB Johnston (2016). Weedon’s Skin Pathology Essentials E-Book. New York: USA, Elsevier Health Sciences. Retrieved from
  4. NM Bouvier, P Palese (2008).  The biology of influenza viruses. Vaccine.
  5. K Rogers, RJ Kadner (2018). Bacteria. Retrieved from Encyclopedia Britannica.


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