Virus Classification

Study Notes

Viruses are infectious, obligate intracellular parasites made up of genetic materials (DNA or RNA) covered by a protein coat, sometimes a membrane (e.g., Corona virus has a membrane surrounding the nucleic acid).
Viruses exist everywhere, infect all living things, and we carry viral genomes as part of our own genetic materials.
Most viruses do not carry any disease (e.g., Polyomavirus).

Viruses are not living things; they do not think, grow, or respond to stimuli.
Viruses are passive agents of infection, chemical reactants to their environment, and organisms with dual phases (virion and infected host living cell).

Microscopic (100-1,000 smaller than human cells), infectious agents that replicate inside living host cells.
Non-living (lack metabolic processes, cells, energy production, and growth/response to stimuli).
Contain either DNA or RNA protected by a capsid (protein coat) or host cell membrane.
Obligate intracellular parasites that depend on host cells to replicate and reproduce.
Infect specific host cells, with each virus targeting specific cell types.
Reproduce by attaching to host cells, penetrating host cells' membrane, replicating genetic particles, assembling new viral particles, and releasing new particles from the host cell.
Many are pathogenic, causing diseases in humans, animals, plants, and bacteria (e.g., Flu, Common cold, HIV/AIDS, Hepatitis, COVID-19).
Have high mutation rates and genetic recombination, impacting vaccine and antiviral treatment effectiveness.
Host cells' antibodies and immune cells neutralize and clear infections, and vaccines stimulate immune system to produce specific antibodies against viruses.

Based on core content (single-stranded or double-stranded DNA/RNA), shape (complex, radial symmetry, cubical/icosahedral, rod/spiral/helical symmetry), mode of replication (cytoplasm, nucleus, or both), phenotypic characteristics, chemical composition, capsid structure, and size.
Examples include Picornavirus, Parvovirus, Adenovirus, Herpes virus, Reovirus, Human Poliovirus, Hepatitis A virus, Influenza virus, and Rabies virus.

Based on host cell type (animal, plant, bacterial, insect cells).
Based on mode of transmission (airborne, contaminated water/food, sexual, blood, insect bites).