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 .
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 . 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 
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 . 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 . 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 . Common diseases caused by protozoans are malaria, amoebic dysentery, giardia, toxoplasmosis, and African trypanosomiasis (African sleeping sickness).
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 . 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.