An infectious disease is caused by a pathogenic agent. Non-infectious conditions include many inherited disorders and diseases associated with environmental or lifestyle factors. Not every microorganism is pathogenic. Bacteria are prokaryotic cells; fungi and protists are eukaryotic. Some parasitic animals, including worms, also cause disease.
Viruses are non-cellular agents containing nucleic acid in a protein coat, sometimes with a lipid envelope. They depend on host cells to reproduce. Prions are infectious misfolded proteins and contain no nucleic-acid genome. A disease being infectious does not mean direct contact is its only transmission route: water, food or vectors may transmit the agent. Classification concerns the agent, not just the symptoms.
Example 1
An agent has DNA, ribosomes and no membrane-bound nucleus. Which pathogen group fits?
A bacterium fits this prokaryotic cellular organisation.
Example 2
An infectious agent consists of misfolded protein and has no nucleic-acid genome. Classify it.
It is a prion, rather than a virus or a cellular microorganism.
Watch out:Many are harmless or beneficial; a pathogen is an agent capable of causing disease in a susceptible host.
Pathogens and disease categories practice | Year 11 Biology | Ascent Arcade