Which organisms are classified as pathogenic?

Pathogenic organisms are those that can cause disease in humans, animals, or plants. This classification primarily includes microorganisms such as bacteria, viruses, and protozoan cysts. These pathogens can lead to a range of health issues, from mild infections to severe diseases, depending on their nature and the susceptibility of the host.

Bacteria can cause diseases like tuberculosis and strep throat; viruses are responsible for illnesses such as influenza and COVID-19; and protozoan cysts, such as those from Giardia, can result in gastrointestinal diseases. The ability of these organisms to invade the host, replicate, and provoke an immune response is what categorizes them as pathogenic.

In contrast, organisms classified in the other options do not cause disease. The first option describes pretend organisms used for demonstrations, which are not relevant in the context of disease. The second option refers to plants that purify water, highlighting their role in environmental health rather than causing illness. Lastly, while fungi are essential in decomposition and nutrient cycling, they are typically not classified as pathogenic unless they specifically cause disease, which is not the primary function described here. Thus, the organisms that can harm health and are classified as pathogenic are exclusively those that include bacteria, viruses, and cysts that can