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Characterizing and Classifying Viruses, Viroids, and Prions

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1 Characterizing and Classifying Viruses, Viroids, and Prions
Chapter 13 Characterizing and Classifying Viruses, Viroids, and Prions

2 Characteristics of Viruses
Minuscule, acellular infectious agent having either DNA or RNA Causes many infections of humans, animals, plants, and bacteria Causes most of the diseases that plague the industrialized world 10/15/11 MDufilho . 2

3 Characteristics of Viruses
Cannot carry out any metabolic pathway Neither grow nor respond to the environment Cannot reproduce independently Recruit the cell’s metabolic pathways to increase their numbers No cytoplasmic membrane, cytosol, organelles (with one exception) Have extracellular and intracellular state 10/15/11 MDufilho 3

4 Characteristics of Viruses
Extracellular State Called virion Protein coat (capsid) surrounding nucleic acid Nucleic acid and capsid also called nucleocapsid Some have phospholipid envelope Outermost layer provides protection and recognition sites for host cells Intracellular State Capsid removed Virus exists as nucleic acid 10/15/11 MDufilho 4

5 Figure 13.1 Virions-overview
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6 Characteristics of Viruses
Genetic Material of Viruses Show more variety in nature of their genomes than do cells Primary way scientists categorize and classify viruses May be DNA or RNA, but never both dsDNA, ssDNA, dsRNA, ssRNA Linear and segmented or single and circular Much smaller than genomes of cells 10/15/11 MDufilho 6

7 Figure 13.2 The relative sizes of genomes
Partial genome of E. coli Viral genome 10/15/11 MDufilho

8 Characteristics of Viruses
Hosts of Viruses Most viruses infect only particular host’s cells Affinity of viral surface proteins for proteins on host cell May be so specific they infect only particular kind of cell in a particular host Generalists – infect many kinds of cells in many different hosts 10/15/11 MDufilho 8

9 Figure 13.3 Hosts of viral infections-overview
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10 Figure 13.4 Sizes of selected virions
E. coli (bacterium) (1000 nm  3000 nm) Red blood cell (10,000 nm in diameter) Bacterial ribosomes (25 nm) Smallpox virus (200 nm  300 nm) Poliovirus (30 nm) Bacteriophage T4 (50 nm  225 nm) Bacteriophage MS2 (24 nm) Tobacco mosaic virus (15 nm  300 nm) 10/15/11 MDufilho

11 Characteristics of Viruses
Capsid Morphology Capsids Provide protection for viral nucleic acid Means of attachment to host’s cells Composed of proteinaceous subunits called capsomeres Capsomere made of single or multiple types of proteins Three basic shapes - helical,polyhedral,complex 10/15/11 MDufilho 11

12 Figure 13.5 The shapes of virions-overview
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13 Figure 13.6 Bacteriophage T4-overview
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14 Characteristics of Viruses
The Viral Envelope Acquired from host cell during viral replication or release Envelope is portion of membrane system of host Composed of phospholipid bilayer and proteins Some proteins are virally coded glycoproteins (spikes) Envelope’s proteins and glycoproteins often play role in host recognition 10/15/11 MDufilho 14

15 Figure 13.7 Enveloped virion-overview
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16 Table 13.2 Families of Human Viruses
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17 Viral Replication Lysogeny Modified replication cycle
Infected host cells grow and reproduce normally for generations before they lyse Temperate phages Prophages – inactive phages Lysogenic conversion results when phages carry genes that alter phenotype of a bacterium 10/15/11 MDufilho 17

18 Replication of Animal Viruses
Viral Replication Replication of Animal Viruses Same basic replication pathway as bacteriophages Differences result from Presence of envelope around some viruses Eukaryotic nature of animal cells Lack of cell wall in animal cells 10/15/11 MDufilho 18

19 Replication of Animal Viruses
Viral Replication Replication of Animal Viruses Attachment of animal viruses Chemical attraction Animal viruses do not have tails or tail fibers Have glycoprotein spikes or other attachment molecules that mediate attachment Uncoating Direct penetration Membrane fusion 10/15/11 MDufilho 19

20 Replication of Animal Viruses
Viral Replication Replication of Animal Viruses Synthesis of animal viruses Requires different strategy depending on its nucleic acid DNA viruses often enter the nucleus RNA viruses often replicate in the cytoplasm Must consider How mRNA is synthesized What serves as template for nucleic acid replication 10/15/11 MDufilho 20

21 Figure 13.13 Synthesis of proteins and genomes in animal RNA viruses-overview
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22 Replication of Animal Viruses
Viral Replication Replication of Animal Viruses Assembly and release of animal viruses Most DNA viruses assemble in nucleus Most RNA viruses develop solely in cytoplasm Number of viruses produced depends on type of virus and size and initial health of host cell Enveloped viruses cause persistent infections Naked viruses are released by exocytosis or lysis 10/15/11 MDufilho 22

23 Figure 13.14 The process of budding in enveloped viruses
Enveloped virion Budding of enveloped virus Viral glycoproteins Cytoplasmic membrane of host Viral capsid 10/15/11 MDufilho

24 Replication of Animal Viruses
Viral Replication Replication of Animal Viruses Latency of animal viruses When animal viruses remain dormant in host cells May be prolonged for years with no viral activity Some latent viruses do not become incorporated into host chromosome Incorporation of provirus into host DNA is permanent 10/15/11 MDufilho 24

25 The Role of Viruses in Cancer
Animal’s genes dictate that some cells can no longer divide or are prevented from unlimited division Genes for cell division “turned off” or genes inhibiting division “turned on” Neoplasia Uncontrolled cell division in multicellular animal Mass of neoplastic cells is tumor Benign vs. malignant tumors Metastasis Cancers 10/15/11 MDufilho 25

26 Figure 13.16 The oncogene theory of the induction of cancer in humans
Normal state: DNA Protooncogene Gene for repressor Represses mRNA Repressor Result: No cancer First “hit”: Virus inserts promoter DNA Oncogene Gene for repressor Represses mRNA Repressor Result: Still no cancer Second “hit”: Virus inserts into represssor gene DNA Oncogene No repressor protein because gene is segmented mRNA 10/15/11 MDufilho Protein Causes cell division Result: Cancer

27 The Role of Viruses in Cancer
Environmental factors that contribute to the activation of oncogenes Ultraviolet light Radiation Carcinogens Viruses 10/15/11 MDufilho © 2012 Pearson Education Inc. 27

28 The Role of Viruses in Cancer
Viruses cause 20–25% of human cancers Some carry copies of oncogenes as part of their genomes Some promote oncogenes already present in host Some interfere with tumor repression Specific viruses are known to cause ~15% of human cancers Burkitt’s lymphoma Hodgkin’s disease Kaposi’s sarcoma Cervical cancer 10/15/11 MDufilho 28

29 Some consider them complex pathogenic chemicals
Are Viruses Alive? Some consider them complex pathogenic chemicals Others consider them the least complex living entities Use sophisticated methods to invade cells Have the ability to take control of their host cell Are able to replicate themselves 10/15/11 MDufilho 29

30 Other Parasitic Particles: Viroids and Prions
Characteristics of Viroids Extremely small, circular pieces of RNA that are infectious and pathogenic in plants Similar to RNA viruses, but lack capsid May appear linear due to H bonding 10/15/11 MDufilho 30

31 Genome of bacteriophage T7 PSTV
Figure The RNA strand of the small potato spindle tuber viriod (PSTV) Genome of bacteriophage T7 PSTV 10/15/11 MDufilho

32 Figure 13.21 One effect of viroids on plants
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33 Other Parasitic Particles: Viroids and Prions
Characteristics of Prions Proteinaceous infectious agents Cellular PrP protein Made by all mammals Normal structure with -helices called cellular PrP Prion PrP Disease-causing form with -pleated sheets called prion PrP Prion PrP changes shape of cellular PrP so it becomes prion PrP 10/15/11 MDufilho 33

34 Other Parasitic Particles: Viroids and Prions
ANIMATION Prions: Overview 10/15/11 MDufilho 34

35 Other Parasitic Particles: Viroids and Prions
Characteristics of Prions Normally, nearby proteins and polysaccharides force PrP into cellular shape PrP mutations result in formation of prion Pr 10/15/11 MDufilho 35

36 Other Parasitic Particles: Viroids and Prions
ANIMATION Prions: Characteristics 10/15/11 MDufilho 36

37 Other Parasitic Particles: Viroids and Prions
Characteristics of Prions Prion diseases Fatal neurological degeneration, fibril deposits in brain, and loss of brain matter Large vacuoles form in brain Characteristic spongy appearance Spongiform encephalopathies Prions only destroyed by incineration or autoclaving in 1 N NaOH 10/15/11 MDufilho 37

38 Other Parasitic Particles: Viroids and Prions
ANIMATION Prions: Disease 10/15/11 MDufilho 38

39 Viral Diseases of the Skin and Wounds
Herpes Infections Signs and symptoms Slow spreading skin lesions Recurrence of lesions is common Pathogen and virulence factors Caused by human herpesviruses 1 and 2 Produce various proteins that act as virulence factors Pathogenesis Painful lesions caused by inflammation and cell death Cause fusion of cells to form syncytia 10/15/11 MDufilho

40 Figure 19.11 Oral herpes lesions
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41 Viral STDs Genital Herpes Signs and symptoms
Small blisters on or around the genitals or rectum Pathogen and virulence factors Human herpesvirus 2 causes most cases Virus can become latent in nerve cells Pathogenesis Herpesvirus kills epithelial cells at infection site Blisters may form at sites far from initial infection site Babies can become infected during birth 10/15/11 MDufilho

42 Figure 24.12 Herpes lesions of the eyes and skin-overview
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43 Viral Diseases of the Skin and Wounds
Herpes Infections Epidemiology Spread between mucous membranes of mouth and genitals Herpes infections in adults are not life threatening Diagnosis, treatment, and prevention Diagnosis made by presence of characteristic lesions Immunoassay reveals presence of viral antigens Chemotherapeutic drugs help control the disease but do not cure it 10/15/11 MDufilho

44 Viral Diseases of the Skin and Wounds
Warts Benign epithelial growths on the skin or mucous membranes Can form on many body surfaces Various papillomaviruses cause warts Most warts are harmless Transmitted via direct contact and fomites Diagnosed by observation Various techniques to remove warts New warts can develop due to latent viruses 10/15/11 MDufilho

45 Figure 19.13 Various kinds of warts--lesions caused by papillomaviruses-overview
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46 Figure Genital warts 10/15/11 MDufilho

47 Viral STDs Genital Warts Signs and symptoms Pathogen
Warts on the genitalia and surrounding areas Large growths called condylomata acuminata may form Pathogen Caused by human papillomaviruses (HPV) HPV can cause various cancers 10/15/11 MDufilho

48 Viral STDs Genital Warts Pathogenesis and epidemiology
HPVs invade skin or mucous membranes during sex Most common STD in the U.S. Diagnosis, treatment, and prevention Diagnosis made by presence of warts Variety of methods available to remove warts Vaccine available against HPV strain associated with cervical cancer 10/15/11 MDufilho

49 Viral Diseases of the Skin and Wounds
Chickenpox and Shingles Signs and symptoms Chickenpox characterized by lesions on the back and trunk that spread across body Shingles lesions localized to skin along an infected nerve Pathogen Varicella-zoster virus (VZV) causes both diseases Pathogenesis Infected dermal cells cause rash characteristic of chickenpox Virus becomes latent in nerve ganglia Reactivated VZV causes shingles 10/15/11 MDufilho

50 Viral Diseases of the Skin and Wounds
Chickenpox and Shingles Epidemiology Chickenpox occurs mostly in children Disease is more severe in adults Risk of shingles increases with age Diagnosis, treatment, and prevention Diagnosis based on characteristic lesions Treatment based on relief of symptoms Vaccine available against chickenpox 10/15/11 MDufilho

51 Viral Diseases of the Skin and Wounds
Other Viral Rashes Erythema infectiosum Caused by an erythrovirus of family Parvoviridae Respiratory disease that manifests as a rash Also referred to as fifth disease Roseola Caused by human herpesvirus 6 (HHV-6) Characterized by a rose-colored rash Coxsackievirus infection Caused by coxsackie A viruses Produces lesions like those from herpes infections Also causes hand-foot-and-mouth disease 10/15/11 MDufilho © 2012 Pearson Education Inc.

52 Figure 19.16 A case of erythema infectiosum (fifth disease)
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53 Viral Diseases of the Nervous System
Viruses more readily cross the blood-brain barrier Occur more frequently than bacterial and fungal infections Include meningitis, polio, rabies, and encephalitis 10/15/11 MDufilho

54 Viral Diseases of the Nervous System
Viral Meningitis Signs and symptoms Similar to those of bacterial meningitis Usually milder than those of bacterial or fungal meningitis Pathogens and virulence factors 90% of cases caused by viruses in the genus Enterovirus Pathogenesis Damage to cells in the meninges triggers meningitis 10/15/11 MDufilho

55 Viral Diseases of the Nervous System
Viral Meningitis Epidemiology More common than bacterial and fungal meningitis Spread via respiratory droplets and feces Diagnosis, treatment, and prevention Diagnosed by characteristic signs and symptoms in the absence of bacteria in the CSF No specific treatment exists 10/15/11 MDufilho

56 Viral Diseases of the Nervous System
Arboviral Encephalitis Arboviruses are arthropod-borne viruses Transmitted via blood-sucking arthropods (e.g., mosquitoes) Mosquito-borne arboviruses can cause arboviral encephalitis As zoonotic diseases, they rarely affect humans Arboviruses usually cause mild, coldlike symptoms Can cause if cross the blood-brain barrier 10/15/11 MDufilho

57 Table 20.2 Characteristics of Arboviral Encephalitis Diseases and Viruses in the United States
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58 Figure 20.15 Transmission of six encephalitis arboviruses
Humans can be infected via mosquito bites. Small mammals are hosts for VEE and California viruses only. Encephalitis arboviruses can overwinter inside mosquito eggs. Mosquitoes are vectors. Wild birds Horses, and rarely other domestic mammals are hosts for equine viruses. Domestic fowls Birds are hosts for all six encephalitis arboviruses. 10/15/11 MDufilho

59 Figure 20.16 Human West Nile virus encephalitis in the United States
Reported cases Annual deaths (299) Number of reported cases (124) (264) (43) (86) (30) (84) (9) Time (months/years) 10/15/11 MDufilho

60 Viral Diseases of the Nervous System
Arboviral Encephalitis Diagnosis based on signs and symptoms Confirmed by presence of arbovirus-specific antibodies in CSF Treatment is supportive Prevention involves limiting contact with mosquitoes Use netting and insect repellents Eliminate stagnant water Vaccines for horses available against EEE, WEE, VEE, and WNV 10/15/11 MDufilho

61 Viral Cardiovascular and Systemic Diseases
Infectious Mononucleosis Signs and symptoms Severe sore throat and fever occur initially Followed by swollen lymph nodes, fatigue, appetite loss Pathogen and virulence factors Epstein-Barr virus (EBV or HHV-4) is the causative agent EBV establishes latent infection in host EBV implicated in number of other diseases 10/15/11 MDufilho

62 Figure 21.13 Diseases associated with Epstein-Barr virus
Diseases of EBV Oral hairy leukoplakia* Burkitt’s lymphoma (shown) Chronic fatigue syndrome* Nasopharyngeal cancer Asymptomatic Infectious mononucleosis State of cellular immunity Lacking Poor Normal Vigorous *EBV implicated, not proven 10/15/11 MDufilho

63 Viral Cardiovascular and Systemic Diseases
Infectious Mononucleosis Pathogenesis and epidemiology Transmission occurs via saliva EBV infects B lymphocytes Majority of adults have antibodies against EBV Diagnosis, treatment, and prevention Diagnosed by presence of large, lobed B lymphocytes and neutropenia Treatment focuses on relieving symptoms Prevention is difficult since EBV occurrence is widespread 10/15/11 MDufilho

64 Viral Cardiovascular and Systemic Diseases
Cytomegalovirus Disease Signs and symptoms Asymptomatic in most cases Complications in neonates and immunodeficient individuals Pathogen and virulence factors Caused by Cytomegalovirus Pathogenesis and epidemiology Transmit by direct contact with body fluids or transplacentally One of the most common infections of humans Diagnosis, treatment, and prevention Fomivirsen is administered for eye infections No vaccine is available 10/15/11 MDufilho

65 “Owls eye” cell 10/15/11 MDufilho
Figure An abnormally enlarged “owl’s eye” cell indicates Cytomegalovirus (CMV) infection “Owls eye” cell 10/15/11 MDufilho

66 Viral Cardiovascular and Systemic Diseases
African Viral Hemorrhagic Fever Signs and symptoms Fever and fatigue Minor petechiae progress to severe internal hemorrhaging Pathogens and virulence factors Caused by Ebolavirus or Marburgvirus Pathogenesis and epidemiology Occurs primarily in Africa Transmitted via contact with bodily fluids of infected individual 10/15/11 MDufilho

67 Figure 21.17 Filamentous Ebolavirus
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68 Marburg Ebola 10/15/11 MDufilho
Figure Sites in which locally acquired cases of Marburg and Ebola viruses have occurred Marburg 10/15/11 MDufilho Ebola

69 Viral Cardiovascular and Systemic Diseases
African Viral Hemorrhagic Fever Diagnosis, treatment, and prevention Diagnosis based on characteristic symptoms and presence of virus in the blood Treatment involves fluid and electrolyte replacement Vaccines are being studied for their effectiveness in humans 10/15/11 MDufilho

70 Viral Diseases of the Upper Respiratory System
Common Cold Signs and symptoms Sneezing, runny nose, congestion, sore throat, malaise, and cough Pathogens and virulence factors Enteroviruses (rhinoviruses) are the most common cause Numerous other viruses cause colds Pathogenesis Cold viruses replicate in and then kill infected cells 10/15/11 MDufilho

71 Viral Diseases of the Upper Respiratory System
Common Cold Epidemiology Rhinoviruses are highly infective Spread by coughing/sneezing, fomites, or person-to-person contact Diagnosis, treatment, and prevention Signs and symptoms are usually diagnostic Pleconaril can reduce duration of symptoms Hand antisepsis is important preventive measure 10/15/11 MDufilho

72 Viral Diseases of the Lower Respiratory System
Influenza Signs and symptoms Sudden fever, pharyngitis, congestion, cough, myalgia Pathogens and virulence factors Influenza virus types A and B are the causative agents Mutations in hemagglutinin and neuraminidase produce new strains Occurs via antigenic drift and antigenic shift 10/15/11 MDufilho

73 Figure 22.12 Influenzavirus budding from a cell
Neuraminidase Hemagglutinin Envelope ssRNA molecule in helical capsid 10/15/11 MDufilho

74 Viral Diseases of the Lower Respiratory System
Influenza Mutations in hemagglutinin and neuraminidase produce new strains Occurs via antigenic drift and antigenic shift Named by type (A or B), location and date or original identification Example – A/Singapore/1/80 (H1N2) If isolated from an animal, that is included Hong Kong flu or swine flu Asia is major site of antigentic shift 10/15/11 MDufilho

75 Figure 22.13 The development of new strains of flu viruses-overview
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76 Figure 22.11 A scene from the flu pandemic of 1918-19
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77 Viral Diseases of the Lower Respiratory System
Influenza Pathogenesis Symptoms produced by the immune response to the virus Flu patients are susceptible to secondary bacterial infections Virus causes damage to the lung epithelium Epidemiology Transmitted via inhalation of viruses or by self-inoculation Complications occur most often in the elderly, children, and individuals with chronic diseases 10/15/11 MDufilho

78 Viral Diseases of the Lower Respiratory System
Influenza Diagnosis, treatment, and prevention Signs and symptoms during a community-wide outbreak are often diagnostic Treatment involves supportive care to relieve symptoms Oseltamivir and zanamivir can be administered early in infection Prevent by immunization with a multivalent vaccine 10/15/11 MDufilho

79 Viral Diseases of the Lower Respiratory System
Severe Acute Respiratory Syndrome (SARS) Signs and symptoms High fever, shortness of breath, and difficulty breathing Later develop dry cough and pneumonia Pathogen and virulence factors Caused by a coronavirus called SARS virus Pathogenesis and epidemiology SARS virus spreads via respiratory droplets Diagnosis, treatment, and prevention Diagnosis based on signs and symptoms of SARS Treatment is supportive 10/15/11 MDufilho

80 Figure The face of SARS 10/15/11 MDufilho

81 Viral Diseases of the Lower Respiratory System
Respiratory Syncytial Virus Infection Most common childhood respiratory disease Signs and symptoms Fever, runny nose, and coughing in babies or immunocompromised individuals Mild coldlike symptoms in older children and adults Pathogen Respiratory syncytial virus (RSV) Pathogenesis Virus causes syncytia to form in the lungs Immune response to RSV further damages the lungs 10/15/11 MDufilho

82 Multinucleated syncytium
Figure A syncytium forms when RSV triggers infected cells to fuse with uninfected cells Multinucleated syncytium Newly infected cell Nucleus RSV Infected host cell Uninfected cell Newly infected cell Infected host cell RSV Multinucleated syncytium 10/15/11 MDufilho

83 Viral Diseases of the Lower Respiratory System
Respiratory Syncytial Virus Infection Epidemiology Transmission occurs via close contact with infected persons Diagnosis, treatment, and prevention Diagnosis made by immunoassay Supportive treatment for young children Prevention includes aseptic technique of health care and day care employees 10/15/11 MDufilho

84 Viral Diseases of the Digestive System
Viral Gastroenteritis Signs and symptoms Similar to bacterial gastroenteritis Pathogens and pathogenesis Caused by caliciviruses, astroviruses, and rotaviruses Epidemiology More cases occur in winter Diagnosis, treatment, and prevention Serological test distinguishes among viruses Treatment is based on fluid and electrolyte replacement Vaccine for rotavirus exists 10/15/11 MDufilho

85 Figure 23.13 Some viruses causing gastroenteritis-overview
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86 Figure 23.14 Deaths from rotaviral diarrhea are most common in developing countries
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87 Viral Diseases of the Digestive System
Viral Hepatitis Signs and symptoms Jaundice, abdominal pain, fatigue, vomiting, appetite loss Symptoms may occur years after initial infection Host immune response causes much of the liver damage Pathogen and pathogenesis Hepatitis A virus (HAV) Hepatitis B virus (HBV) Hepatitis C virus (HCV) Hepatitis delta virus (HDV) Hepatitis E virus (HEV) 10/15/11 MDufilho

88 Table 23.2 Comparison of Hepatitis Viruses
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89 Viral Diseases of the Digestive System
Viral Hepatitis Diagnosis, treatment, and prevention Initial diagnosis made by observation of jaundice, enlarged liver, or fluid in the abdomen Serological testing can identify viral antigens HBV diagnosed by presence of viral proteins in body fluids Supportive care for symptoms Prevent with good hygiene and protected sex or abstinence Vaccines are available against HAV and HBV 10/15/11 MDufilho

90 Figure 23.15 The three types of viral particles produced by hepatitis B viruses-overview
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