Bacteria are single celled, prokaryotic organisms that are capable of living in extreme conditions and have cell walls composed of peptidoglycan. They are either heterotrophic or autotrophic and can be harmless or cause disease (Mycobacterium tuberculosis).

Bacteria

Viruses are simple microorganisms that infect cells with the potential to cause disease. A virus particle, or virion, is made of RNA or DNA and an outer protein shell called a capsid. Viruses are dependent on an infected host cell in order to reproduce; they cannot reproduce by themselves.

Viruses

A parasite is an organism that takes from and hurts its host organism in order to survive. 

Parasite

The name Fungi encompasses a diverse group of heterotrophic organisms both macroscopic, like mushrooms and molds, and microscopic such as yeasts and spores.

Fungi

Georgetown University

In infectious disease epidemiology, an agent is an infectious organism or pathogen, such as a virus, bacterium, paraiste, etc. that can cause disease. 
Some factors that can influence an agent's ability to cause disease include: pathogenicity, virulence, addictive qualities, infectivity, ability to survive outside body, susceptibility to antibiotics, etc. 


Agent/Pathogen

“Vector-Borne Diseases.” World Health Organization, World Health Organization, www.who.int/news-room/fact-sheets/detail/vector-borne-diseases. 

Vector Borne Diseases

Centers for Disease Control and Prevention. (n.d.). Centers for Disease control and Prevention. Centers for Disease Control and Prevention. https://www.cdc.gov/. 

Centers for Disease Control and Prevention

"a sign is any objective evidence of a disease that can be observed by others (for example a skin rash or lump)."

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