2017 Pearson Education Inc Introduction A virus is an obligate intracellular parasite Viruses enter a host cell and use the host s biosynthetic machinery to reproduce and synthesize its proteins ID: 726304
Download Presentation The PPT/PDF document "Viruses Chapter 33 Roadmap." is the property of its rightful owner. Permission is granted to download and print the materials on this web site for personal, non-commercial use only, and to display it on your personal computer provided you do not modify the materials and that you retain all copyright notices contained in the materials. By downloading content from our website, you accept the terms of this agreement.
Slide1
VirusesSlide2
Chapter 33 Roadmap.
© 2017 Pearson Education, Inc.Slide3
Introduction
A
virus is an obligate, intracellular parasite Viruses enter a host
cell
and use the host’
s biosynthetic machinery to reproduce and synthesize its proteins
Most biologists would argue that viruses are not alive, because they depend on their host cell to satisfy the key attributes of lifeSlide4
Introduction
Viruses
Have a genomeAre well adaptedEvolve Nearly all organisms examined thus far are parasitized by at least one kind of virusSlide5
Table 33.1
© 2017 Pearson Education, Inc.Slide6
Why Do Biologists Study Viruses?
The enormous diversity of potential host cells has resulted in the evolution of an equally diverse assortment of viruses
Do viruses contribute to organismal evolution?Slide7
Viruses Shape the Evolution of Organisms
Viruses have promoted evolution of organismal traits
Physical barriers and immune defenses that reduce the occurrence or impact of infectionViruses have also directly influenced the genetic makeup of organisms
Lateral gene transfer
—introduce foreign genes into cellular genomes by picking up genes from one organism and shuttling them to anotherSlide8
Viruses Shape the Evolution of Organisms
Viruses also contribute their own genetic material to organisms
From 5% to 8% of the human genome consists of remnants of viral genomes from past infectionsSome viral genes are part of what makes us human
A protein encoded by an abandoned viral gene is necessary for development and function of placentaSlide9
Figure 33.1
Central nervous system
dengue virus
polio virus
rabies virus
West Nile virus
Lymphatic and
immune systems
Epstein-Barr virus
HIV
measles virus
Trachea and lungs
adenovirus
influenza virus
SARS-CoV
Heart
Coxsackie virus
Liver and digestive tract
dengue virus
Ebola virus
hepatitis A, B, C, D, E
viruses
rotavirus
yellow fever virus
Blood vessels and
blood cells
Ebola virus
hantavirus
Reproductive organs
herpes simplex 2 virus
papillomavirus
Skin
herpes simplex 1 virus
measles virus
papillomavirusrubella virussmallpox virusvaricella zoster virus
Peripheral nerves
rabies virusvaricella zoster virus
© 2017 Pearson Education, Inc.Slide10
Viruses Cause Disease
An
epidemic is a disease that rapidly infects a large number of individuals over a widening area Viruses have caused the most devastating epidemics in recent human history
A
pandemic
is a worldwide epidemic
The
“
Spanish flu
”
outbreak of 1918–1919 was the most devastating pandemic to date
The strain of influenza virus that emerged in 1918 was particularly
virulent
(it tended to cause severe disease)
Worldwide, the Spanish flu killed up to 50 million peopleSlide11
Figure 33.2
(a)
Deadly impact of the 1918 influenza pandemic
Life expectancy (years)
This downward spike is due
to the 1918 flu epidemic
Year
(b)
Emergency hospital at the start of the 1918 pandemic
© 2017 Pearson Education, Inc.Slide12
Current Viral Pandemics in Humans: AIDS
Human immunodeficiency virus
(HIV) causes acquired immune deficiency syndrome
(
AIDS
)
AIDS has surpassed the 1918–1919 influenza,
as well as
measles, and smallpox epidemics, in its impact on humansSlide13
How Does HIV Cause AIDS?
HIV parasitizes and destroys cells called
helper T cells of the immune system, the body
’
s defense system against disease
During HIV infection, the total number of helper T cells in the blood gradually declines
The number of T cells the body manufactures cannot keep up with the number of T cells the virus destroysSlide14
Figure 33.3
Helper T cells per mm
3
blood
Acute
phase
Weeks
Rapid
decline
Chronic phase
AIDS
Gradual decline
Years
© 2017 Pearson Education, Inc.Slide15
How Does HIV Cause AIDS?
When the T-cell count drops, the immune system’
s responses to invading bacteria and viruses become less and less effectiveHIV kills people indirectly—it makes them susceptible to pneumonia, parasites, and unusual types of cancer
Slide16
What Is the Scope of the AIDS Pandemic?
AIDS has already killed 36 million people worldwide
The highest rates of infection in sub-saharan Africa
23% of the Botswana population is HIV positive
There are about 34 million HIV-infected people worldwide
An additional 2.3 million people are infected
each year
Because HIV is primarily a sexually transmitted virus, it disproportionately affects young adultsSlide17
How Do Biologists Study Viruses?
Researchers who study viruses usually focus on how to prevent or reduce the effects of the diseases that they cause
To treat viral infections, need to identify virus specific targets and then use
Vaccines—activate the immune system against the targets
Antiviral drugs—directly interfere with viral replicationSlide18
How Do Biologists Study Viruses?
The first step in studying a virus is to isolate it, which takes researchers into the realm of
nanobiologyStructures are measured in billionths of a meter—
1 nanometer (nm) is 10
–9
meter
Viruses range in size from only about 20 to 300 nm in diameter
Viruses are tiny relative to eukaryotic or even bacterial cellsSlide19
Figure 33.4
Eukaryotic cell
(human red blood cell)
7
µ
m
Bacterial cell
(
E. coli
)
2
µ
m
0.1
µ
m
Virus particles
(HIV)
© 2017 Pearson Education, Inc.Slide20
How Do Biologists Study Viruses?
Researchers use Koch’
s postulates to isolate a virus and confirm that it is the causative agent of infectionOnce a virus is isolated, biologists analyze
The structure of the
virion
The nature of the genetic material that is transmitted from one host to another
Mechanism by which the virus replicatesSlide21
Analyzing Morphological Traits
In terms of overall structure, most viruses fall into just two general categories:
Those enclosed only by a protein shell called a capsid
Those
enclosed by both a capsid
and one or more membranous
envelopesSlide22
Analyzing Morphological Traits
Most viruses produce
virions with helical or icosahedral capsidsSome viruses have more complex capsid shapesThe capsid serves two functions:
Protects the genome while outside the host
Releases the genome when infecting a new cell Slide23
Figure 33.5
(a)
Tobacco mosaic virus
(b)
Adenovirus
(c)
Bacteriophage T4
(d)
Smallpox virus
50 nm
100 nm
100 nm
Genome
Protein capsid
Genome
Protein capsid
Genome
Genome
Protein capsid
Protein
capsid
Membranous envelopes
50 nm
© 2017 Pearson Education, Inc.Slide24
Analyzing the Genetic Material
In addition to morphology, viruses can be categorized based on the nature of their genome
Many viruses break the central dogma of molecular biologyInformation flows from DNA →
mRNA → proteinsSlide25
Analyzing the Genetic Material
There is a wide diversity of viral genome types
They may consist of DNA or RNAThey may be linear or circularThey may consist of a single molecule or have several different segments
They may be single stranded or double strandedSlide26
Analyzing the Genetic Material
Single-stranded RNA genomes can be classified as
Positive-sense virusesThe genome contains the same sequences as the mRNA required to produce viral proteins
Negative-sense viruses
The base sequences in the genome are complementary to those in viral mRNAs
Ambisense
viruses
Contain one positive-sense region and one
negative-sense regionSlide27
Analyzing the Genetic Material
Viruses infect their host cells in one of two ways:
Via replicative growth
Produces the next generation of
virions
Often kills the host cell
In a dormant manner
Suspends
virion
production
Allows the virus to coexist with the host for a period
of timeSlide28
Analyzing the Phases of Replicative Growth
Six phases are common to replicative growth in virtually all viruses:
Attachment to a host cell and entry into the cytosol
Viral genome transcription and viral protein production
Viral genome replication
Assembly of a new generation of
virions
Exit from the infected cell
Transmission to a new hostSlide29
Analyzing the Phases of Replicative Growth
A
bacteriophage is a virus that infects bacterial cellsReplicative growth of a bacteriophage is called the lytic cycle
It ends with lysis (destruction) of the cell
In a single cycle, one virus can produce many progeny
The capacity of a bacteriophage to replicate results in nonlinear growthSlide30
Figure 33.6
Virion
Host-cell
genome
DNA mRNA Protein
1.
Viral genome enters host cell.
2.
Viral genome
is transcribed; viral
proteins are produced.
DNA
Protein
6.
Free
virions
in tissue or
environment
are transmitted
to new host.
3. Viral genome is replicated.
5.
Particles exit to exterior.
4.
Particles assemble inside host.
© 2017 Pearson Education, Inc.Slide31
Figure 33.7
Number of cells or viruses
Production of
bacteriophage
Division of cells
Time (minutes)
© 2017 Pearson Education, Inc.Slide32
How Do Viruses Enter a Cell?
The replicative cycle of a virus begins when a free
virion enters a target cellMost plant viruses are inserted directly into the host cell when a sucking insect has disrupted the cell wall
Viruses that parasitize bacterial or animal cells gain entry by binding to a specific molecule on the cell wall or plasma membrane
The viral genome can then be uncoated
At the cell surface
In the endosome Slide33
How Do Viruses Enter a Cell?
Research showed that HIV particles can enter cells only if the
virions bind to a membrane protein called CD4
The hypothesis was that CD4 functions as the receptor for HIV attachment
Researchers predicted that if CD4 is blocked, then HIV will not be able to enter host cells
Experimental tests supported this hypothesis
CD4
antibodies
blocked HIV from binding to and entering the T cell
Only cells with CD4 on their surface can be infected by HIVSlide34
Figure 33.8
HIV uses CD4 as the receptor to enter helper T cells.
Thus, only cells with free, unbound CD4 on their surface can be
infected by HIV.
HIV
Antibody
CD4
protein
Other proteins:
CD4:
Many cells infected No cells infected
1. Start with helper T cells.
2. Add antibodies
to block one
membrane protein in each sample.
3. Add HIV.
×160
Antibody to
protein
other than
CD4
Antibody
to CD4
Add
HIV
Add
HIV
CD4 is the membrane protein HIV uses to enter cells.
CD4 is not the membrane protein HIV uses to
enter cells. Does CD4 protein function as the
receptor that HIV uses to enter host cells? HIV will not infect cells with antibody to CD4 but will
infect other cells.
HIV will infect cells withantibody to CD4.
© 2017 Pearson Education, Inc.Slide35
How Do Viruses Enter a Cell?
HIV virions can enter cells only if the virions bind not only to CD4 but also to a second membrane protein, called a
co-receptorWhen the proteins in a virion’
s envelope bind to both receptors:
The lipid bilayers of the particle
’
s envelope and the plasma membrane of the helper T cell fuse
The viral capsid enters the cytoplasm and infection proceedsSlide36
Figure 33.9a
HIV
CD4
Immune
system cell
Co-receptor
2.
HIV’s envelope protein binds
to CD4 and a co-receptor.
3.
The binding event causes
the membranes to fuse,
allowing the viral capsid to
enter the host cell and start
an infection cycle.
1.
Both HIV particles and
human immune cells have
specialized proteins in their
membranes (just some of
the proteins are shown).
© 2017 Pearson Education, Inc.Slide37
How Do Viruses Enter a Cell?
When the virus is uncoated in the endosome, it is first internalized into the host cell via endocytosis
The endosome vesicle that engulfs the virion acidifies, changing the viral attachment proteinsThese proteins promote fusion of the viral envelope and the endosomal membraneSlide38
Figure 33.9b
H
+
Endosome
Influenza
H
+
Animal cell
H
+
H
+
1.
Influenza’s envelope proteins
attach to a common carbohydrate
found on animal cells, inducing
virion
uptake via endocytosis.
2.
Protons are pumped into
the endosome, causing the
envelope proteins to change
shape.
3.
The envelope and endosome
membranes are brought together
and eventually fuse, releasing the
viral genome into the cytoplasm.
© 2017 Pearson Education, Inc.Slide39
How Do Viruses Enter a Cell?
Researchers are searching for compounds that block attachment and uncoating of viruses
Drugs that interfere with viral infection or replication are called antiviralsSlide40
How Do Viruses Produce Proteins?
In all viruses, translation of viral transcripts depends entirely on the host
Viruses lack ribosomes, amino acids, ATP, and most of the other biosynthetic machinery required for translationSlide41
How Do Viruses Produce Proteins?
Viral mRNAs that code for envelope proteins are
Translated as if they were mRNAs for the cell’s own membrane proteinsmRNAs that code for capsid proteins are
Translated by free cytosolic ribosomes as if they were cellular mRNAs for cytosolic proteinsSlide42
How Do Viruses Produce Proteins?
In some viruses, long polypeptides called
polyproteins are cut into functional proteins by a viral proteaseThe discovery that HIV produces a protease triggered a search for drugs that would inhibit the enzyme
Several HIV protease inhibitors are currently
being used to interfere with viral replicationSlide43
Figure 33.10
(a)
HIV’s protease enzyme
(b)
Could a drug block the active site?
Protease
inhibitor
Active site
of protease
© 2017 Pearson Education, Inc.Slide44
How Do Viruses Copy Their Genomes?
In addition to transcription and translation, viruses must also copy their genetic material to make a new generation of virions
They depend on the host cell for nucleotide monomersSome DNA viruses depend on the host-cell DNA polymerase machinery to replicate their genomesSlide45
How Do Viruses Copy Their Genomes?
Viruses with an RNA genome must supply their own enzyme to make copies of their genome
Most RNA viruses use an RNA polymerase called RNA replicase, which synthesizes RNA from an
RNA template
Synthesis uses ribonucleotides provided by the
host cellSlide46
Figure 33.11
(+)
ssRNA
Replicase
Genomic strand
dsRNA intermediate
Template strand
Replicase
(+)ssRNA products
(genome copies)
© 2017 Pearson Education, Inc.Slide47
How Do Viruses Copy Their Genomes?
Some RNA viruses, such as HIV, are
retroviruses
In retroviruses, the RNA genome is transcribed to DNA by a viral enzyme called
reverse transcriptase
Reverse transcriptase is a DNA polymerase that
Makes a single-stranded
complementary DNA
(
cDNA
) from a single-stranded RNA template
Removes the RNA strand and synthesizes the complementary DNA strand, resulting in double-stranded DNA
The
cDNA
copy of the genome is then inserted by a viral protein into the host-cell chromosomeSlide48
Figure 33.12
cDNA
RNA template
First, reverse transcriptase
synthesizes
cDNA
from RNA
Double-stranded cDNA
cDNA
template
Then, reverse transcriptase
synthesizes double-stranded
DNA from cDNA
© 2017 Pearson Education, Inc.Slide49
How Are New Virions Assembled?
During assembly, viral genomes are packaged into capsids
Some viruses also include copies of non-capsid proteins inside the capsidThe assembly process is not yet well understoodSome viruses assemble the capsid first and then use motor proteins to pull the viral genome inside
Other viruses assemble the capsid around the genomic materialSlide50
How Are New Virions Assembled?
Enveloped viruses use the host endomembrane system to transport their envelope proteins to the appropriate membrane for assemblySlide51
How Do Progeny Virions Exit an Infected Cell?
Most viruses leave a host cell in one of two ways:
Enveloped viruses bud from a cell through the cell membrane
They take part of the cell membrane with them, including envelope proteins that were inserted into the membrane
Nonenveloped
viruses burst out of the cell
Release their
virions
from the cell by lysing it—commonly referred to as the burst
Bacteriophages produce lytic enzymes that break down host-cell wallSlide52
Figure 33.13
(a)
An enveloped virus budding from a host cell
Cell interior
Cell exterior
Protease
activity
Viral core
Viral envelope proteins
(b)
Nonenveloped
viruses bursting from a host cell
Cell wall
Virions
escape
from lysed cell
Plasma membrane
50 nm
500 nm
© 2017 Pearson Education, Inc.Slide53
How Are Virions Transmitted to New Hosts?
A new replicative cycle begins when the virions infect new cells—either within the same organism or in another organism
A virus’s long-term success depends on its ability to be transmitted through the environment from one individual to another
Natural selection thus favors viral alleles that allow the virus to
Replicate within a host
Be transmitted to new hostsSlide54
Analyzing How Viruses Coexist with Host Cells
All viruses undergo replicative growth, but some can arrest the replicative cycle and enter a dormant state
In bacteriophages, this alternate type of infection is called lysogeny
No virions are produced during lysogeny
However, the host
’
s DNA polymerase replicates the viral DNA each time the cell divides
Molecular cues from the host cell can direct the virus toward lysogeny or the lytic cycleSlide55
Figure 33.14
Infection
Integration
Replication
of genome
4.
Cell divides. Viral genome is
transmitted to daughter cells.
OR
DNA mRNA Protein
1.
Viral genome
enters host cell.
2.
Viral genome
integrates into host-
cell genome.
3.
Host-cell DNA
polymerase copies
chromosome.
At any point after integration,
the virus may activate the
replicative cycle.
© 2017 Pearson Education, Inc.Slide56
Analyzing How Viruses Coexist with Host Cells
In viruses that infect animal cells, the dormant state is called
latency HIV has a latency periodViral genes are not expressed until the helper T cell host is activated by the immune responseSlide57
What Themes Occur in the Diversification of Viruses?
Mutation and natural selection guarantee that viral genomes will continually adapt to their hosts
’ defensesBecause most viral polymerases have high error rates and viruses lack error repair enzymes, mutation rates are extremely high
Many viruses change constantly—giving them the potential to evolve rapidlySlide58
Where Did Viruses Come From?
Nobody knows how viruses originated
Biologists are considering three hypotheses to explain the origin of viruses:Origin in plasmids and transposable elements?
Origin in symbiotic bacteria?
Origin at the origin of life?Slide59
Origin in Plasmids and Transposable Elements?
Hypothesis: Simple viruses are “
escaped gene sets” Mobile genetic elements descended from genes that escaped from prokaryotic or eukaryotic chromosomes
Escaped gene sets encode information needed to replicate themselves at expense of genomes that once held them
Included instructions for making a capsid
Evidence: Discovered virus recently derived from intact prokaryotic or eukaryotic genes, and viral genome still strongly resembled those genesSlide60
Origin in Symbiotic Bacteria?
Hypothesis: DNA viruses descend from free-living bacteria that lived inside eukaryotic cells
The bacteria gradually degenerated into viruses Slowly lost genes needed to synthesize ATP, nucleic acids, amino acids, and other compoundsEvidence to support: Discovery of a cell that possesses a genome similar to that of a large DNA virusSlide61
Origin at the Origin of Life?
Hypothesis: Viruses descended from the first RNA-based life-forms on Earth
Evidence to support: Ubiquity of viruses suggests that they have been evolving along with organisms since life beganSeveral proteins commonly expressed in many viruses are not expressed in any known cell
Genes coding for these proteins may have come from an RNA-world pool of genes instead of a cellSlide62
Emerging Viruses, Emerging Diseases
Although it is not known how the various types of viruses originated, it is certain that viruses will continue to diversify
HIV is an example of a virus responsible for an emerging disease
New illnesses that suddenly affect significant numbers of individuals in a host population
The causative agents are considered to be emerging viruses
They switched from their traditional host species to a new host—humansSlide63
Some Emerging Viruses Arise from Genome Reassortment
Influenza vaccines are updated annually because of small changes that arise from the virus
’s error-prone RNA replicaseHowever, influenza can also acquire alleles that are entirely new to the
strain
A virus strain consists of populations that have similar characteristicsSlide64
Some Emerging Viruses Arise from Genome Reassortment
Influenza has a single-stranded RNA genome consisting of eight segments
Most segments encode only one proteinIf two viruses infect the same cell, replicated genomic segments are randomly shuffled
Progeny often have segments from each parent virusSlide65
Figure 33.15
Influenza
Host cell
Recombinant
strain
1.
Two different strains of
influenza infect the same cell.
2.
Replication produces a
mix of strain-specific genomic
segments in host cytoplasm.
3.
Reassortment
of genomic
segments generates new,
recombinant strains.
© 2017 Pearson Education, Inc.Slide66
Using Phylogenetic Trees to Understand Emerging Viruses
Researchers analyze a virus’
s evolutionary history to determine if an emerging virus has jumped to a new hostFor example, analyzing the phylogenetic tree for HIV reveals three findings:
There are
siminan
immunodeficiency viruses
There are two distinct types of human HIV
HIV-1 and HIV-2
Multiple
“
jumps
”
have occurred
There are several strains of HIV-1, suggesting that it has jumped between species several timesSlide67
Figure 33.16
HIV strains that infect humans
HIV strains that infect other primates
SIV-Sykes monkey
SIV-sooty
mangabey
HIV-2
SIVs from four
monkey species
SIV-chimp strain cpzTAN1
HIV-1 strain O
(“O” stands for outlier)
SIV-chimp strain
cpzUS
HIV-1 strain N
(“N” stands for new;
discovered recently)
HIV-1 strain M
(“M” stands for main
strain responsible for
the AIDS epidemic)
© 2017 Pearson Education, Inc.Slide68
Responding to a Virus Outbreak
A virus outbreak is indicated by a large number of patients:
With identical and unusual disease symptoms In the same geographic area Over a short period of time
Physicians report these cases to public health officialsSlide69
Responding to a Virus Outbreak
Public health officials
Identify the agent that is causing the new illness
Identify the origin of the outbreakSlide70
Responding to a Virus Outbreak
In 2002, outbreak of an unknown respiratory
syndrome started in southern China and spread to several countries across the globe, causing over 8000 probable infections and killing 774 people Illness became known as severe acute respiratory syndrome (SARS)
Caused by a new coronavirus
Genome of SARS-
CoV
was rapidly sequenced
Source of the outbreak still had to be determinedSlide71
Responding to a Virus Outbreak
Epidemiologists had to decide how patients acquired the virus
Horseshoe bat is the natural host (reservoir)Virus also identified in the civet, a small mammal commonly sold in markets in southern China
This observation provided link between bats and human outbreakSlide72
Key Lineages of Viruses
Because scientists are almost certain that viruses originated multiple times, there is no such thing as the phylogeny of all viruses
To organize the diversity of viruses, researchers use the Baltimore classification system:Viruses are grouped into seven general categories based on the nature of their genetic material and how they replicateSlide73
Figure 33.17
DNA viruses
Class I
dsDNA
Class II
ssDNA
Class III
dsRNA
RNA viruses
Class IV
(+)ssRNA
Class V
(–)ssRNA
Reverse-transcribing viruses
Class VI
(+)ssRNA (RT)
Reverse
transcription
Reverse
transcription
Class VII
dsDNA (RT)
mRNA
To viral mRNA and proteins
Genome replication
Proteins
© 2017 Pearson Education, Inc.Slide74
Key Lineages of Viruses
Within the Baltimore classification system, about 100 virus families are distinguished by
Virion and genome morphology
The nature of the host species
How the virus replicates within the hostSlide75
Table 33.2
© 2017 Pearson Education, Inc.Slide76
Double-Stranded DNA (dsDNA) Viruses
Smallpox
Eradicated by worldwide vaccination programsHuman papilloma virus (HPV), herpesviruses, and adenoviruses
Viruses like HPV can induce host cells to enter S phase despite absence of normal growth signals
This ability may be responsible for the proposed link between HPV infections and cervical cancerSlide77
Single-Stranded DNA (ssDNA) Viruses
Canine parvovirus type 2
Causes a fatal disease in dogs if untreatedRare in domesticated dogs due to vaccineDensovirus
Triggered an epidemic of wasting disease in sea stars that is decimating populations from Baja California to southern AlaskaSlide78
Table 33.3
© 2017 Pearson Education, Inc.Slide79
Double-Stranded RNA (dsRNA) Viruses
Many diseases in rice, corn, sugarcane, and other crops are caused by
dsRNA virusesThe bluetongue virusCauses a deadly disease that has significantly affected the livestock industry
Reovirus
and rotavirus
Infections are leading cause of infant diarrheaSlide80
Positive-Sense Single-Stranded RNA
([+]ssRNA
) VirusesMost of the commercially important plant viruses belong to this group
Members of this group also
Parasitize bacteria, fungi, and animals
Include the viruses that cause the common cold, polio, SARS-
CoV
, West Nile virus, and hepatitis in humansSlide81
Negative-Sense Single-Stranded RNA
([–]ssRNA
) VirusesA wide variety of plants and animals are parasitized by viruses that have (–)
ssRNA
genomes
Flu, mumps, and measles
Ebola virusSlide82
Table 33.4
© 2017 Pearson Education, Inc.Slide83
Reverse-Transcribing
(+)ssRNA Viruses
Rous sarcoma virus (chickens), mouse mammary tumor virus, and murine (mouse) leukemia virus
Contribute to the development of cancer
HIVSlide84
Reverse-Transcribing dsDNA Viruses
Hepatitis B virus
About 5% of adults infected with HBV have a chronic infection that has been linked to the development of liver cancer