1. Definition and Properties of a Virus
Viruses are filterable agents.
Viruses are obligate intracellular parasites.
Viruses cannot make energy or proteins independently of a host cell.
Viral genomes may be RNA or DNA but not both.
Viruses have a naked capsid or an envelope morphology.
Viral components are assembled and do not replicate by "division."

2. Consequences of Viral Properties
Viruses are not living.
Viruses must be infectious to endure in nature.
Viruses must be able to use host cell processes to produce their components (viral messenger RNA, protein, and identical copies of the genome).
Viruses must encode any required processes not provided by the cell.
Viral components must self-assemble.

3. Means of Classification and Naming of Viruses
Structure: size, morphology, and nucleic acid (e.g., picornavirus [small RNA], togavirus)
Biochemical characteristics: structure and mode of replication*
Disease: encephalitis and hepatitis viruses, for example
Means of transmission: arbovirus spread by insects, for example
Host cell (host range): animal (human, mouse, bird), plant, bacteria
Tissue or organ (tropism): adenovirus and enterovirus, for example

5. Picornavirus
Togavirus
Retrovirus
Poxvirus
Adenovirus
Reoviruses
Norwalk virus
Coxsackie virus

6.            
                                                                                                                                                                                                                          
The DNA viruses and their morphology. The viral families are determined by the structure of the genome and the morphology of the virion

7.            
                                                                                                                                                                                                                          
The RNA viruses, their genome structure, and their morphology. The viral families are determined by the structure of the genome and the morphology of the virion. E, enveloped; N, naked capsid.

8. Viruses nm
Parvovirus 18nm
Poxvirus 300nm

10. Families of DNA Viruses and Some Important Members Family Members
POXVIRIDAE Smallpox virus, vaccinia virus, monkeypox, molluscum contagiosum
Herpesviridae Herpes simplex virus types 1 and 2, varicella- zoster virus, Epstein-Barr virus, CMV, human herpesviruses 6, 7, and 8
Adenoviridae Adenovirus
Papilloma viridae Papilloma virus
Polyoma viridae JC virus, BK virus,SV40
Hepadnaviridae Hepatitis B virus
Parvoviridae Parvovirus B19, adeno-associated virus *The italicized virus is the important, or prototype, virus for the family.

13. Steps in Viral Replication Recognition of the target cell Attachment
Penetration
Uncoating
Macromolecular synthesis
Early messenger RNA (mRNA) and nonstructural protein synthesis: genes for enzymes and nucleic acid-binding proteins
Replication of genome
Late mRNA and structural protein synthesis
Post-translational modification of protein
Assembly of virus
Budding of enveloped viruses
Release of virus

15. Examples of Viral Attachment ProteinsVirus Family Virus VAP
Picornaviridae Rhinovirus VP1-VP2-VP3
Adenoviridae Adenovirus Fiberprotein
Reoviridae Reovirus σ Rotavirus VP7
Togaviridae Semliki Forest v E1-E2-E3 complex Rhabdoviridae Rabies virus G protein
Orthomyxoviridae Influenza A HA
Paramyxoviridae Measles virus HA
Herpesviridae Epstein-Barr v gp350 and gp220
Retroviridae Murine leukemia vgp Human immunodeficiency virusgp120
gp, glycoprotein; HA, hemagglutinin; VAP, viral attachment protein.

16. Examples of Viral Receptors
Virus Target Cell Receptor*
Epstein-Barr virus B cell C3d complement receptor CR2 (CD21)
HIV Helper T cell CD4 molecule and chemokine coreceptor Rhinovirus Epithelial cells ICAM-1 (immunoglobulin superfamily protein)
Poliovirus Epithelial cells Immunoglobulin superfamily protein
Herpes simplex virus Many cells Herpesvirus entry mediator (HVEA),nectin-1
Rabies virus Neuron Acetylcholine receptor, NCAM (neural cell adhesion molecule)
Influenza A virus Epithelial cells Sialic acid
B19 parvovirus Erythroid precursors Erythrocyte P antigen (globoside)
*Other receptors for these viruses may also exist. ICAM-1, Intercellular adhesion molecule.

17. Mechanisms of Viral Pathogenesis
Determinants of Viral Disease
Nature of the Disease
Target tissue
Portal of entry of virus
Access of virus to target tissue
Tissue tropism of virus
Permissiveness of cells for viral replication
Viral pathogen (strain)
Severity of Disease
Cytopathic ability of virus
Immune status
Competence of the immune system
Prior immunity to the virus
Immunopathology
Virus inoculum size
Length of time before resolution of infection
General health of the person
Nutrition
Other diseases influencing immune status
Genetic makeup of the person
Age

18.            
                                                                                                                     
The stages of viral infection. The virus is released from one person, is acquired by another, replicates, and initiates a primary infection at the site of acquisition. Depending on the virus, it may then spread to other body sites and finally to a target tissue characteristic of the disease. B, The cycle starts with acquisition, as indicated, and proceeds until the release of new virus. The thickness of the arrow denotes the degree to which the original virus inoculum is amplified on replication. The boxes indicate a site or cause of symptoms. C, Time course of viral infection. The time course of symptoms and the immune response correlate with the stage of viral infection and depend on whether the virus causes symptoms at the primary site or only after dissemination to another (secondary) site. CMV, cytomegalovirus; HBV, hepatitis B virus; HIV, human immunodeficiency virus.

19. Mechanisms of Viral Pathogenesis
Failed infection (abortive infection).
Cell death (lytic infection).
Replication without cell death (persistent infection).

20. Determinants of Viral Pathogenesis
Interaction of Virus with Target Tissue
Access of virus to target tissue
Stability of virus in the body
Temperature
Acid and bile of the gastrointestinal tract
Ability to cross skin or mucous epithelial cells (e.g., cross the gastrointestinal tract into the bloodstream)
Ability to establish viremia
Ability to spread through the reticuloendothelial system
Target tissue
Specificity of viral attachment proteins
Tissue-specific expression of receptors
Cytopathologic Activity of the Virus
Efficiency of viral replication in the cell
Optimum temperature for replication
Permissiveness of cell for replication
Cytotoxic viral proteins
Inhibition of cell's macromolecular synthesis
Accumulation of viral proteins and structures (inclusion bodies)
Altered cell metabolism (e.g., cell immortalization)
Host Protective Responses
Antigen-nonspecific antiviral responses
Interferon
Natural killer cells and macrophages
Antigen-specific immune responses
T-cell responses
Antibody responses
Viral mechanisms of escape of immune responses
Immunopathology
Interferon: flulike systemic symptoms
T-cell responses: delayed-type hypersensitivity
Antibody: complement, antibody-dependent cellular cytotoxicity, immune complexes
Other inflammatory responses