Viruses are fascinating microorganisms that blur the line between living and non-living. They're incredibly tiny parasites that hijack cells to reproduce, lacking the machinery for independent life. Their simple yet diverse structures allow them to infect a wide range of hosts.

Viral characteristics set them apart from other microbes. They're much smaller than bacteria, can't grow or divide on their own, and have simpler genetic structures. Understanding these properties is key to grasping how viruses spread, cause disease, and evolve.

Virus Properties and Characteristics

Fundamental Nature of Viruses

• Viruses function as non-cellular, obligate intracellular parasites requiring host cells for replication
• Viruses lack organelles, ribosomes, and cellular machinery needed for independent metabolism and reproduction
• Viral replication occurs by hijacking host cell machinery to produce viral components and assemble new virus particles
• Viruses exist in two states
  • Extracellular virion form (inactive)
  • Intracellular replicative form (active)

Viral Structure and Genome

• Basic virus structure comprises genetic material (DNA or RNA) enclosed within a protein coat called a capsid
• Viral genomes exhibit diversity
  • Single-stranded or double-stranded
  • Composed of either DNA or RNA, never both
  • Can be segmented or non-segmented
  • May contain additional structures (5' cap, 3' poly-A tail)
• Viruses display extreme size diversity, ranging from ~20 nanometers to over 1000 nanometers in diameter (bacteriophages, mimiviruses)

Viruses vs Other Microorganisms

Distinguishing Characteristics

• Viruses lack cellular structure and independent reproductive ability, unlike living organisms (bacteria, fungi, protozoa)
• Viruses measure significantly smaller than most microorganisms, with the largest viruses smaller than the smallest bacteria (poxviruses vs mycoplasmas)
• Viruses contain nucleic acids and possess more complex structures compared to prions (infectious proteins)
• Viruses differ from genetic elements like plasmids and transposons
  • Can exist outside host cells
  • Have mechanisms for cell entry and exit

Unique Viral Properties

• Viruses do not grow or undergo cell division, instead assembling from pre-formed components
• Viral genetic structure appears much simpler than cellular organisms, often containing only a few genes
• Viruses can infect organisms across all domains of life, including other microbes (bacteriophages, virophages)

Virus Structure and Components

Viral Capsid and Envelope

• Viral capsid functions as a protein shell encapsulating and protecting the viral genome
• Capsid shapes vary
  • Icosahedral (adenoviruses)
  • Helical (tobacco mosaic virus)
  • Complex (bacteriophages)
• Some viruses possess an envelope
  • Lipid bilayer derived from host cell membranes
  • Contains viral proteins and glycoproteins
  • Examples include influenza virus and HIV

Viral Proteins and Enzymes

• Structural proteins form the capsid and other viral structures
• Non-structural proteins involve in viral replication and host cell interactions
• Some viruses contain enzymes within their virions
  • Reverse transcriptase in retroviruses
  • RNA-dependent RNA polymerase in some RNA viruses (influenza)
• Viral surface proteins (spike proteins) play crucial roles in host cell recognition and attachment (SARS-CoV-2)

Structural Complexity

• Viral structure complexity varies greatly
  • Simple non-enveloped viruses (poliovirus)
  • Complex viruses with multiple capsid layers and elaborate surface structures (poxviruses)

Virus Infectivity and Host Specificity

Virus Infectivity Concepts

• Virus infectivity describes the ability of a virus to
  • Enter a host cell
  • Replicate within the host
  • Produce infectious progeny virions
• Viral infectivity quantification uses measures such as
  • Plaque-forming units (PFU)
  • Tissue culture infectious dose (TCID50)

Host Specificity and Tropism

• Host specificity defines the range of host species or cell types a virus can infect
• Determined by
  • Presence of specific receptors on host cells
  • Corresponding viral attachment proteins
• Tissue tropism explains why certain viruses preferentially infect specific tissues or organs within a host organism (hepatitis B virus targets liver cells)
• Host range varies
  • Narrow (infecting one or few closely related species)
  • Broad (infecting a wide range of hosts across different taxonomic groups)

Zoonotic Viruses and Host Factors

• Zoonotic viruses cross species barriers, infecting both animals and humans (rabies virus, influenza A virus)
• Often lead to emerging infectious diseases
• Factors influencing host specificity include
  • Viral surface proteins
  • Host cell receptors
  • Intracellular factors required for viral replication
  • Host's immune response