Covers the basic nature of viruses as obligate intracellular, acellular particles and their structure, from nucleic acid core and capsid to lipid envelope. Also outlines classification criteria, including the seven-class Baltimore system, a table of essential properties, the stepwise replication cycle, host cell effects, laboratory cultivation, detection methods, inactivation and prevention strategies.
Topic 86 of 113 in the MU Sofia syllabus for Microbiology (Dental Medicine, Year 2). The free AceMed note covers it in 11 sections, about 469 words (4 min of reading), with 15 flashcards.
In this note
Viruses – Nature and Properties
I. Definition and General Nature
II. Viral Structure
III. Classification of Viruses
IV. Essential Properties
V. Viral Replication Cycle
VI. Effects of Viruses on Host Cells
VII. Laboratory Cultivation
VIII. Detection and Diagnosis
IX. Resistance and Inactivation
X. Prevention and Control
I. Definition and General Nature
Viruses are obligate intracellular parasites — they can only replicate inside living host cells.
Composed of nucleic acid (DNA or RNA) enclosed in a protein coat (capsid); some have a lipid envelope.
Do not have cellular structures (no ribosomes, mitochondria, or nucleus).
Acellular, non-living outside the host — no metabolism or independent replication.
Measured in nanometers (20–300 nm range).
Specificity: infect specific hosts and often specific tissues (tropism) within the host.
II. Viral Structure
Nucleic Acid Core:
Either DNA or RNA, never both.
DNA: single-stranded (ss) or double-stranded (ds); RNA: ss or ds; positive-sense (+) or negative-sense (−).
Can be linear or circular, segmented (e.g., influenza) or non-segmented.
Capsid:
Protein coat that encloses the genome.
Made of capsomeres.
Shapes: icosahedral, helical, or complex (e.g., poxviruses).
Function: protection and attachment.
Envelope (in some viruses):
Lipid bilayer derived from the host cell membrane.
Embedded with viral glycoproteins (spikes) important for host cell entry and immune evasion.
Enveloped viruses are more sensitive to drying, heat, and detergents.
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