Education, Epidemiology

What is the Agent and Host Model in Infectious Diseases?

Author Chandana Balasubramanian , 18-Oct-2022

All infectious diseases have something in common. Every disease is caused by an agent and needs one or more hosts that get infected by the disease. Even as our understanding of infectious diseases continues to expand, the building blocks of epidemiology stay the same. Learning begins with the study of agents and hosts. 

What is an Agent?


An agent is any microorganism that causes a disease. The 6 agents of disease are bacteria, viruses, fungi, protozoa, helminths (worms), and prions. Some people prefer to group protozoa and helminths under the term parasites, leaving 5 broad categories of agents [1]. 



Bacteria are single-celled microorganisms found all over the planet. In fact, bacteria are assumed to be the first living organism on earth. Some bacteria are harmful to humans, while others are beneficial. Every single one of us has trillions of microorganisms in our bodies. An average 200-pound (~90 kg) adult has almost 2-66 pounds (1-13 kg) of bacteria in them [2]. There are many categories of bacteria, but one way to classify them is by their shape: 

  • Cocci: Spherical bacteria 
  • Bacilli: Rod-shaped bacteria
  • Spirilla: Spiral-shaped bacteria
  • Vibrios: Comma-shaped bacteria
  • Spirochaetes: corkscrew-shaped bacteria [3]


Commonly known bacterial infections include pneumonia, food poisoning, sepsis, urinary tract infections, and strep throat. 



Viruses are infectious microorganisms that contain DNA or RNA, surrounded by a protein coat. Viruses straddle the border between life and death. They contain DNA or RNA and multiply, which makes them life-like, but they cannot act independently. They need a host to replicate [4]. Viruses are tiny, much smaller than bacteria, and only seen under powerful microscopes. Many common viral infections are known to humans, including ebola, influenza, chickenpox, AIDS, measles, COVID-19, pneumonia, dengue, and herpes

Note: Bacteria, viruses, and fungi can cause pneumonia.



Fungi (singular: fungus) include yeasts, mold and mildew, mushrooms, and more. Fungi are eukaryotes, which means their cells are bound by a membrane and have a well-defined nucleus. They are found everywhere – in the air around us, in soil and water, and even in our bodies. Fungi can even grow inside cancerous tumors. There are about 144,000 known species of fungi [5]. Common fungal infections in humans include candidiasis, dermatophytosis (tinea), blastomycosis, coccidioidomycosis (valley fever), aspergillosis, mucormycosis, and pneumocystis pneumonia. 



Protozoa are single-celled organisms found worldwide, particularly in moist habitats like fresh water and soil. Some protozoa are parasites that live and reproduce in animal and human bodies, causing diseases. Amoeba, giardia, and plasmodium are examples of parasitic protozoans [6]. Common diseases caused by protozoans are malaria, amoebic dysentery, giardia, toxoplasmosis, and African trypanosomiasis (African sleeping sickness). 

Helminths (worms)


The term ‘helminth’ simply means ‘worm,’ but many are parasitic and can cause diseases in animals and humans. The most common parasitic helminths are categorized based on their shape:

  • Flatworms or platyhelminths
  • Thorny-headed worms or acanthocephalans
  • Roundworms or nematodes


These worms are one of the world’s most common parasites and are often quite large. They can even range to about a meter or more (slightly bigger than a yard). Usually, they are transmitted to humans when we eat or drink their eggs or larvae through contaminated food or water. Some helminths require an intermediate host before they infect humans [7]. Common helminth infections in humans include ascariasis, Acanthocephala, enterobiasis (pinworm infection), Guinea worm disease, and lymphatic filariasis. 



Prions are a type of protein that trigger abnormalities in normal proteins in the brain. Prion diseases are rare but deadly, with a high fatality rate. The term ‘prion’ stands for ‘proteinaceous infectious particle’ because scientists initially believed that prions were mysterious infectious agents that contained only protein (without DNA or RNA in them). However, more recent research showed that prions are distorted proteins found in normal cell membranes that could induce other proteins in the brain to change [8,9]. Research on prions has accelerated because they are responsible for rare but fatal human diseases, like Creutzfeldt-Jakob disease (CJD), kuru, and Gerstmann-Straussler-Scheinker (GSS) disease. 


What is a Host of Disease?


A host is a living organism that allows a disease-causing agent to live in them. A host may also nourish and support the pathogen, giving them a space to survive and reproduce. Infectious disease hosts can be humans, animals, plants, or other living beings. Hosts do not have to be infected by the disease to spread a disease-causing agent to other hosts. There are different types of hosts, broadly classified as follows:

  • Primary hosts
  • Secondary hosts


Primary Hosts


The primary host, also known as the definitive host, harbors a disease-causing agent till the agent reaches adulthood and can reproduce (sexually, when applicable). For example, the primary host for plasmodium, the agent that causes malaria, is a female Anopheles mosquito. Another example is chickenpox, where humans are the definitive (and only) hosts for the Varicella-Zoster virus.

Secondary Hosts


Secondary hosts are intermediate hosts. They entertain an agent for a specific part of the agent’s developmental cycle in a short period. Let’s continue to use malaria as an example. Plasmodium (the agent) reproduces sexually in the female mosquito and only then infects humans. Since humans do not host the pathogen during its reproductive cycle, we are intermediate hosts. Birds are primary hosts for avian flu (bird flu); humans can be secondary hosts, though human infections are rare. 


Why is the Agent-Host Model Important in Epidemiology?


The agent-host model is fundamental to the world of epidemiology. Any knowledge of infectious diseases always begins with identifying the causative agent(s) and their hosts. 


What are the Limitations of the Agent-Host Model?


The agent-host model is an excellent start to understanding epidemiology but is very basic. For instance, simply knowing that influenza is caused by the influenza virus and that humans can be hosts does not help us lower infection rates. The environment responsible for the spread of the disease is a huge missing piece of the puzzle. For the flu, we need to know that people in close quarters can spread the flu and that the virus thrives in colder temperatures, and more. Only then can we develop an effective public health response. An evolution of the agent-host model is the agent-host-environment model, also known as the epidemiological triad or epidemiological triangle. 


Frequently Asked Questions


Is a pathogen the same as an agent?


Yes, pathogens are infectious agents of disease. The term ‘pathogen’ is mainly used to describe microbes like bacteria, fungi, viruses, and more, while larger disease-causing organisms like protozoa and helminths are called parasites. An agent is an overarching catch-all term for all disease-causing organisms, regardless of their size. 

What is the difference between a host and a vector?


A vector is any organism that transmits infectious agents from one species to another. A host is any organism that allows an agent to live in them for a brief period or its entire development lifecycle. For example, the Aedes mosquito is a vector that transmits the dengue virus to humans. In this example, mosquitoes are vectors and intermediate hosts. However, humans are only hosts for dengue and not vectors since they do not transmit the virus to other species. 


Teaching the Epidemiological Triangle


As a teacher, how do you engage students in learning about the agent-host model or agent-host-environment mode (epidemiological triad)? 

The epidemiological triangle is one of the foundational learning elements of epidemiology. Learning to connect diseases with their agents, hosts, and environment can be highly challenging for students new to epidemiology. The best way to learn is by doing. Educators, GIDEON offers a lesson plan on the epidemiological triangle to help you train students with exercises on diseases and their agent-host-environment models. Other GIDEON lesson plans include: 


Use GIDEON lesson plans and case studies to help enhance your courses’ infectious disease content. 


[1] Healthcare workers: Infectious agents,”, 29-Jul-2021. [Online].

[2] NIH Human Microbiome Project defines normal bacterial makeup of the body,” National Institutes of Health (NIH), 31-Aug-2015. [Online]. 

[3] Microbiology Society, “Bacteria,” [Online].

[4] J. Segre, “Virus,” [Online].

[5] Royal Botanic Gardens, Kew, “State of the world’s fungi 2018,” 2018.

[6] R. G. Yaeger, “Protozoa: Structure, classification, growth, and development,” in Medical Microbiology. 4th edition, University of Texas Medical Branch at Galveston, 1996.

[7] D. Wakelin, “Helminths: Pathogenesis and defenses,” in Medical Microbiology. 4th edition, University of Texas Medical Branch at Galveston, 1996.

[8] Prion diseases,”, 10-Nov-2021. [Online]. 

[9] Microbiology Society, “Prions,” [Online]. 

Chandana Balasubramanian

Chandana Balasubramanian is an experienced healthcare executive who writes on the intersection of healthcare and technology. She is the President of Global Insight Advisory Network, and has a Masters degree in Biomedical Engineering from the University of Wisconsin-Madison, USA.

Articles you won’t delete.
Delivered to your inbox weekly.