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Note Sheet 16 - Viruses
Swine (H1N1) Flu Viruses
What is a virus?
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Virus – a particle made of
a nucleic acid (DNA or RNA)
surrounded by protein coat
Capsid – the outer coat of
protein that covers a virus
Viruses are extremely small – Millions of them
could fit into one single cell
Viruses are grouped by the type of organisms
they infect – There are 3 main types of viruses:
plant viruses, animal viruses, and bacterial
viruses
Virus Types
Viruses are small
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http://learn.genetics.utah.edu/content/be
gin/cells/scale/
Some viruses cause disease
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Many human diseases are caused by
viruses
Common cold
Flu
AIDS
Rabies
What is the flu?
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Influenza (the flu) is a contagious respiratory illness
caused by influenza viruses
It can cause mild to severe illness, and at times can lead
to death
Each year in the United States on average, 5% to 20%
of the population gets the flu; on average, more than
200,000 people are hospitalized from flu-related
complications, and; about 36,000 people die from flurelated causes
There are many different types of flu virus, or “strains”
that circulate from year to year and make people sick
What is “swine” or “H1N1” flu?
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Swine flu is a new strain of the flu virus
that was first identified passing from
human to human earlier this year
Because this particular virus has never
circulated in human populations before,
nobody has immunity
Many more people than usual have
become, and will become infected with
this particular strain of the virus
Percentage of Visits for Influenza-like Illness (ILI) Reported by the U.S.
Outpatient Influenza-like Illness Surveillance Network (ILINet), National
Summary 2008-2009 and Previous Two Seasons (Week ending 11-1409)
Swine Flu Virus
Swine Flu Virus
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Blue and Red
“spikes” represent
surface proteins
Blue are
hemagglutinin (HA)
proteins
Red are
neuraminidase (NA)
proteins
Flu Virus Names
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There are 16 types of HA protein and 9
types of NA protein
A flu virus has only one type of HA and
one type of NA, so that is how they are
named
Swine flu is grouped as an “H1N1” virus
because it has type 1 HA protein and type
1 NA protein
Is a virus a living thing?
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Viruses do NOT have all the characteristics of life
Viruses are NOT made of cells and do not
have cell parts
 Viruses do NOT take in or use energy
 Viruses CANNOT reproduce by themselves
Although there is some argument, most
biologists do not consider viruses to be living
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Where do viruses come from?
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Viruses enter living
cells and use the cell
parts of the infected
cell to build more
viruses
Host cell – a cell that
has been infected by a
virus
What happens when a virus infects a cell?
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Lytic Cycle – process where a virus enters a cell,
makes copies of itself, and causes the cell to burst,
releasing the new viruses
What happens when a virus infects a cell?
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Lytic Cycle – process where a virus enters a cell,
makes copies of itself, and causes the cell to burst,
releasing the new viruses
What happens when a virus infects a cell?
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Lytic Cycle – process where a virus enters a cell,
makes copies of itself, and causes the cell to burst,
releasing the new viruses
What happens when a virus infects a cell?
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Lytic Cycle – process where a virus enters a cell,
makes copies of itself, and causes the cell to burst,
releasing the new viruses
Flu Attack! How A Virus
Invades Your Body
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http://www.xvivo.net/zirus-antivirotics/
What happens when a virus infects a cell?
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Lysogenic Cycle – process where the
virus injects its DNA into the DNA of the
host cell which is then replicated along
with the host cell’s DNA
 Eventually, the virus DNA will begin
making more viruses (lytic cycle)
Viruses either make more viruses
immediately (lytic cycle) or “hide out” for
a while (lysogenic cycle)
The Common
Cold – What is it?
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The common cold is a group of symptoms in the upper
respiratory tract caused by a large number of different
viruses
Although more than 200 viruses can cause the common
cold, the perpetrator is usually the rhinovirus, which is to
blame for causing 10% to 40% of colds
Also, the coronaviruses cause about 20% of colds
Corona Virus
Rabies Virus
HIV (Human Immunodeficiency Virus)
How do we defend against viruses?
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Viruses attack and
destroy specific types of
cells, causing the
symptoms of particular
diseases (flu, common
cold, AIDS, rabies)
Vaccine – a weakened or killed virus that is injected
into the body so the immune system can recognize the
virus and destroy it
Vaccines only work if they are given before an infection
– once a viral disease is contracted, it is often too late to
control it
Vaccine Success Story - Smallpox
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Smallpox is a highly
infectious, devastating, and
disfiguring disease that is
caused by variola virus
Smallpox is characterized by
numerous pustules
containing infectious virus all
over the body
The fatality rate is more
than one quarter of infected
patients infected by the
most serious form caused by
Variola major
Vaccine Success Story - Smallpox
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The disease killed an estimated 400,000 Europeans each
year during the 18th century
Of all those infected, 20–60% — and over 80% of
infected children — died from the disease
In the United States, from 1843 to 1855 first
Massachusetts, and then other states required smallpox
vaccination
By 1897, smallpox had largely been eliminated from the
United States
In the early 1950s an estimated 50 million cases of
smallpox occurred in the world each year
After successful vaccination campaigns throughout the
19th and 20th centuries, the WHO certified the
eradication of smallpox in December 1979
The last cases of smallpox in the world occurred in an
outbreak of two cases (one of which was fatal) in
Birmingham, England in 1978
Vaccine Success Story - Polio
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Polio is a viral disease, usually affecting children
and young adults, caused by any of three
polioviruses
Polio virus causes inflammation of the motor
neurons of the brain stem and spinal cord
Symptoms include motor paralysis, followed by
muscular atrophy and often permanent
deformities
Polio was one of the most dreaded childhood
diseases of the 20th century
Polio epidemics have crippled thousands of
people, mostly young children; the disease has
caused paralysis and death for much of human
history
Vaccine Success Story - Polio
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By 1910, much of the
world experienced a
dramatic increase in polio
cases and frequent
epidemics became
regular events, primarily
in cities during the
summer months
These epidemics left
thousands of children
and adults paralyzed
The polio vaccines developed by Jonas Salk in 1952 and
Albert Sabin in 1962 are credited with reducing the global
number of polio cases per year from many hundreds of
thousands to around a thousand
Why don’t we have vaccines for all
viral diseases?
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There are many problems inherent in developing
a good protective anti-viral vaccine
Different types of virus may cause similar
diseases -- e.g. the common cold. As a result, a
single vaccine will not be possible against such a
disease
Viruses can change as they circulate (mutation)
Large animal reservoirs. If these occur, reinfection after elimination from the human
population may occur.