Viruses
Are Viruses Living Things?
Characteristics of Living things are…
• Made of cells
• Can reproduce
• Based on a universal genetic code *
• Grow and develop
• Obtain and use materials and energy
• Respond to the environment
• Maintain homeostasis
• As a group, change over time *
Viral Characteristics
•NON-LIVING
•REPRODUCE ONLY WITHIN A HOST CELL by
hijacking the cells’ machinery
•NAMED FOR DISEASE THEY CAUSE OR PLACE
THEY INFECT
Viruses are named for
• The cells they attack
• The disease they cause
18.1 Studying Viruses and Prokaryotes
KEY CONCEPT
Infections can be caused in several ways.
TEKS 4C
Name tells
you this
virus infects
bacteria
Name tells
you this
virus causes
influenza
18.1 Studying Viruses and Prokaryotes
TEKS 4C
Viruses, bacteria, viroids, and prions can all cause
infection.
• Any disease-causing agent is called a pathogen.
1 nanometer (nm) = one
billionth of a meter
100 nm
eukaryotics cells
10,000-100,000 nm
viruses
50-200 nm
prokaryotics cells
200-10,000 nm
viroids
5-150 nm
prion
2-10 nm
18.2 Viral Structure and Reproduction
• Bacteriophages infect bacteria.
capsid
DNA
tail sheath
tail fiber
TEKS 4C
Viral Size
• Viruses are very small. If an average virus
were the size of an average person, a
bacterial cell would be the size of a
dinosaur over ten stories tall.
Click on the picture for virus size activity
18.1 Studying Viruses and Prokaryotes
• A virus is made of DNA or RNA and a protein coat.
– non-living pathogen
– can infect many
organisms
• A viroid is made only
of single-stranded
RNA.
– causes disease in
plants
– passed through
seeds or pollen
TEKS 4C
18.1 Studying Viruses and Prokaryotes
• A prion is made only of proteins.
– causes misfolding of other proteins
– results in diseases of the brain
TEKS 4C
18.2 Viral Structure and Reproduction
KEY CONCEPT
Viruses exist in a variety of shapes and sizes.
TEKS 4C
18.2 Viral Structure and Reproduction
TEKS 4C
How viruses enter our cells
•
•
•
•
The virus has an antigen that mimics a body cell
The antigen will fit in a cells receptor site
The cell allows the virus to enter the cell
A virus is very specific as to what type of cell it infects
Which type of cell will be infected by
the virus shown below?
18.2 Viral Structure and Reproduction
TEKS 4C
Viruses differ in shape and in ways of entering host
cells.
• Viruses have a simple structure.
– genetic material (DNA or RNA)
– capsid, a protein shell
– maybe a lipid envelope, a protective outer coat
enveloped
(influenza)
capsid
nucleic acid
lipid
envelope
helical
(rabies)
Surface proteins
capsid
nucleic acid
surface
proteins
lipid envelope
polyhedral
(foot-and-mouth
disease)
surface
proteins
capsid
nucleic acid
18.2 Viral Structure and Reproduction
• Viruses enter cells in various ways.
– bacteriophages pierce host cells
colored SEM; magnifications:
large photo 25,000; inset 38,000x
TEKS 4C
18.2 Viral Structure and Reproduction
• Viruses enter cells in various ways.
– viruses of eukaryotes also fuse with membrane
TEKS 4C
How do viruses infect cells?
• Once a virus enters a host cell, two
different processes can occur:
– Lysogenic cycle
– Lytic cycle
18.2 Viral Structure and Reproduction
TEKS 4C
Viruses cause two types of infections.
• A lytic infection causes the host cell to burst.
host bacterium
The bacterophage attaches
and injects it DNA into a host
bacterium.
The host bacterium breaks apart,
or lyses. Bacteriophages are able
to infect new host cells.
The viral DNA
forms a circle.
The viral DNA directs the host
cell to produce new viral parts.
The parts assemble into new
bacteriophages.
The virus may enter the
lysogenic cycle, in which the
host cell is not destroyed.
Lytic Cycle
1. viral DNA is injected into the host cell
2. host cell begins to make mRNA from
the viral DNA
3. viral proteins shut down the host cell’s
regular functions
4. cell is used to make thousands of
viruses (DNA and protein coats)
5. host cell bursts (lyses) and releases
viruses to infect other cells
Lytic Cycle
• When a Lytic Virus infects a cell an
immune response will immediately be
triggered.
• Examples:
• The Cold, The Flu
18.2 Viral Structure and Reproduction
TEKS 4C
• A lysogenic infection does no immediate harm.
The prophage may leave the
host’s DNA and enter the
lytic cycle.
The viral DNA is called a prophage
when it combines with
the host cell’s DNA.
Many cell divisions produce a
colony of bacteria infected
with prophage.
Although the prophage is not
active, it replicates along with
the host cell’s DNA.
Lysogenic Cycle
1. viral DNA is injected into the host cell
2. DNA is integrated into the host DNA
3. host cell divides with the viral DNA as
a part of it
4. eventually the viral DNA can be
triggered to separate from the host cell
DNA and pick up with the lytic cycle at
step 2.
18.3
Viral Diseases
Viruses cause many infectious diseases
• There are many examples of viral infections.
– common cold
TEKS 4C
18.3
Viral Diseases
Viruses cause many infectious diseases
• There are many examples of viral infections.
– common cold
– influenza
TEKS 4C
How do RNA viruses work?
• RNA viruses do not have built in
proofreading, so they mutate and evolve
quickly (like the flu).
• A special kind of RNA virus, called a
retrovirus, can cause DNA to be made
from their RNA.
– Certain cancers, and AIDS are caused by
retroviruses.
18.3
Viral Diseases
TEKS 4C
KEY CONCEPT
Some viral diseases can be prevented with vaccines.
18.3
Viral Diseases
TEKS 4C
Vaccines are made from weakened pathogens.
• A vaccine stimulates the body’s own immune response.
• Vaccines prepare the immune system for a future attack.
• Vaccines are the only way to control the spread of viral
disease.
Fighting Disease

Vaccine
–

Weakened or dead version of a pathogen to
trigger an immune response
Edward Jenner
Viral Diseases
Common Cold
 Causes:
 200+ viruses can cause it, including
rhinoviruses
 No evidence for weather causing a
cold
 Symptoms:
 Runny nose, sore throat, headache,
cough
 Treatment:
 Antihistamines, decongestants &
glycerin based cough suppressants
 NSAIDS (ibuprofen)
 Transmission:
 Inhaling drops of mucus full of
rhinovirus
 Touching contaminated surfaces
 Wash your hands!
Influenza “the flu”
 Cause: influenza (Types A,B & C)
 Symptoms:
 Fever, headache, fatigue, body aches,
congestion
 Treatment:
 CDC currently recommends NOT taking
medications for flu due to growing virus
resistance.
 Transmission:
 Inhaling drops of mucus full of influenza
virus
 Touching contaminated surfaces
 Status:
 Vaccines present, BUT it changes yearly
(RNA virus)
 Threat of flu pandemics worldwide
Smallpox
 Cause: Variola
 Symptoms:
 High fever, body aches, small raised
bumps all over body
 Treatment:
 no proven treatment for smallpox
 Most recover, 30% mortality
 Transmission:
 inhaling droplets of affected saliva
 face-to-face contact with an infected
person
 Status:
 Vaccine present
 Can be fatal
 Eradicated worldwide, last naturally
occurring in 1977
Herpes Simplex I

Cause: (HSV-1)
– Herpes Simplex Virus Type 1

Symptoms:
– Cold sores around mouth
Treatment:
– Topical medication to reduce outbreaks
– Antiviral medication to reduce number of
outbreaks
Transmission:
– Kissing, eating/drinking after one another
Status:
– No vaccine, no cure!
HSV-1



Herpes Simplex II

Cause: (HSV-2)
–

Symptoms:
–

medication to reduce symptoms and reduce
number of outbreaks
Transmission:
–

Pain, itching, blisters on genitals, anus and
thighs
Treatment:
–

Herpes Simplex Virus Type 2
Sexually transmitted, may not be showing
signs, oral or vaginal sex
Status:
–
–
No vaccination, no cure!
1 out of 5 adolescents and adults have had
HSV-2 in the US!
Perianal
HSV-2
Human Papillomavirus (HPV-warts)

Cause: >100 strains of Human Papillomavirus (HPV)

Symptoms:
–
–
–

Treatment:
–
–

Topical creams are available, doctors may freeze or burn them off
May remove warts but virus is still present in the body and warts may return
Transmission:
–
–

Raised or flat, single or multiple swellings on any genital surface, male or female, can
appear cauliflower-like
Can cause cervical cancer in women
No visible signs may occur
Very contagious
sexual contact with infected partner, may not be showing signs
Status:
–
–
–
–
6.2 million new cases a year in U.S.
No cure!
Tests available for presence in women
Gardasil- new vaccine that prevents the 4 highest risk strains of HPV
Hepatitis B
 Cause: Hepatitis B virus (HBV)
 Symptoms:
 Cirrhosis (scarring of the liver), liver cancer, liver
failure, death
 May not look sick
 Treatment:
 Interferon- slows the growth of HBV
 Transmission:
 Any sexual contact
 Blood contact (needles, razors, etc.)
 Status:
 Vaccine is available
 1.25 million American live with chronic hepatitis B Female Cambodian
patient with a distended
Juandice
abdomen due to a
hepatoma resulting from
chronic hepatitis B
infection
Rabies
 Cause: rabies virus
 Symptoms:
 Fever, headache, paralysis, hydrophobia, hallucinations,
foaming at the mouth, ultimately death
 Treatment: no successful treatment
 Transmission:
 bite from an infected mammal (carnivores and bats)
 Status:
 Cases have reduced in domestic animals greatly
 Vaccine is available, only given to people at high risk of
infection
HIV (Human Immunodeficiency Virus)
1.
The HIV virus infects cells of the immune system
called Helper T cells.
a.
Normally Helper T cells activate cells that produce antibodies and
cells that destroy cells infected by a pathogen.
b.
When a Helper T cell is infected by the HIV virus, the immune
system cannot fight off infections from pathogens such as bacteria
and other viruses.
HIV (Human Immunodeficiency Virus)
c.
HIV can cause AIDS (Acquired Immunodeficiency Syndrome) when
enough Helper T cells are infected so the immune system does not
work properly.
d.
People with HIV infections can get sick easily because their
immune system does not work properly. People with AIDS do not
die from the HIV virus, but from these other infections.
HIV (Human Immunodeficiency Virus)


Cause:
– Human Immunodeficiency Virus
Symptoms:
– Fever, headache, tiredness, enlarged lymph nodes
– AIDS (Acquired Immunodeficiency Syndrome)
 weight loss, fatigue, memory loss
– Destroys Helper T cells (part of immune system)
 body cannot fight off illnesses caused by bacteria, other
viruses, fungi, or parasites.
 Ultimately leads to death
HIV (continued)

Treatment:
–

Transmission:
–
–
–
–

sexually (orally, vaginally)
Blood (needles, or through mucus membranes)
Breast milk mother to child
NOT from kissing, touching, insects bites
Status:
–
–

Reverse Transcriptase (RT) inhibitors & Protease Inhibitors- stops the
virus from making copies of itself
No vaccine, several medicines to prevent spread in body
Worldwide: In 2004, 40 million people living with HIV/AIDS, 5 million
newly infected, and 3.1 million deaths
Prevention:
–
Abstinence, Safe sex (Condoms!), avoidance or risky behaviors
associated with sex and drug use
Ebola Hemorrhagic Fever
Electron micrograph
 Cause:
of ebola virus
 1 of 4 Ebola viruses
 Symptoms:
 Fever, headache, muscle/joint pain, red eyes, skin rash
 Diarrhea, vomiting, rash, internal and external bleeding
 Treatment:
 No standard treatment
 Some patients recover but majority die, usually because the lack of
immune response
 Transmission:
 Contaminated body fluids: Blood, mucus, semen, syringes
 Air transmission in monkey Ebola-Reston virus only
 Status:
 Only identified in 1976 with few devastating outbreaks
 RNA virus
Other viral diseases







Polio
Shingles/ chicken pox
Hanta
SARS
Stomach virus (viral gastroenteritis)
Viral meningitis
Rubella, Measles, mumps
Polio
Rubella
Measles