Viruses and Public Health

Viruses are responsible for >50% of
infectious diseases.
 What

are they? Are they alive?
Infectious diseases are those that are
caused by microbes
 Bacteria,
viruses, a few fungi, and a few
protists
 What are significant diseases? How are they
spread? What is our protection?
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Viruses

Most biologists do not consider viruses to
be alive because
 They
are not made of cells.
 They cannot reproduce without a host cell.
 Plus other excuses.

Not made of cells
 This
means that antibiotics which attack cell
function are of no use. Certain drugs work.

Obligate intracellular parasites
 Must
reproduce within a host cell.
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What are viruses made of

Nucleic acid
 Even
though they aren’t cells, they still need a
genetic blueprint so they can reproduce.
 Viruses may have ds DNA, ss DNA, ds RNA,
or ssRNA, depending on the virus.

A covering called a capsid
 A layer
of protein which protects the nucleic
acid and gives the virus its shape.
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Viral size and shape


Viruses range from 30 nm to 300 nm
 Ribosomes are about 30 nm
 The smallest known bacteria are
about 200 nm
Viral shapes:
 helical, polyhedral, and complex
http://www.glencoe.com/qe/images/b136/q4323/ch18_0_a.jpg; www.blc.arizona.edu/.../
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Figures/Icos_Virus.GIF; http://www.foresight.org/Updates/Update48/Images/T4Schematic.jpg
Examples of virus shapes
Ebola
Adenovirus
http://www-cgi.cnn.com/HEALTH/9604/16/nfm/ebola.levine/ebola.reston.large.jpg;
http://www.virology.net/Big_Virology/EM/Adeno-FD.jpg
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Life Cycle of a virus
http://web.uct.ac.za/depts/mmi/jmoodie/flu2life.gif
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Can you catch any virus?
We think that every living thing, bacteria
included, has a virus that infects it.
 But viruses are specific

 Infect
only certain types of organisms
 Infect only certain types of cells
 Attachment requires a match of molecules
between the virus and host cell.
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How do you grow viruses?
Can’t grow them in a Petri dish! Need a
host cell.
 Animal models or human volunteers

 Ethical

Eggs
 In

limits re using humans
bulk for vaccination material
Cell culture
 Viruses
grow on cells living in a dish
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Ways to grow viruses
http://www.fao.org/ag/againfo/foto/egg-facts.gif
news.bbc.co.uk/.../_230333_cell_culture_300.jpg
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How do viruses cause disease?

Viruses damage cells
 Viruses
use host cells energy
 Viruses break open host cells when they
multiply
 Cells die

Your immune system kills infected cells
 White
blood cells called T cells kill infected
cells before too many viruses multiply

Viruses cause birth defects
 Virus
kills important cells in embryo
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Do viruses cause cancer?

Some do.
 Can’t
prove causation because can’t infect
humans with viruses to cause cancer.
 Hepatitis B: liver cancer
 Kaposi’s sarcoma virus: cancer with AIDS
 Papilloma virus: genital warts and cervical
cancer
 Epstein-Bar virus: mononucleosis and
Burkitt’s lymphoma
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Lots of familiar diseases are
caused by viruses
Measles, chicken pox, polio
 Herpes, AIDS, Mono
 SARS, West Nile
 Influenza, smallpox, rabies
 Common cold, Warts
 Parvo, 24 hour stomach “flu”

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How do you catch diseases?
Bacterial, viral, and other microbes

Microbes that cause disease are found
somewhere: a reservoir
 Could
be other humans, could be animals,
could be soil or water.

To cause disease, microbe must get from
the reservoir to you
 If
you are part of the cycle of infection,
microbe must then get to others.
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Transmission


Microbe needs to get from reservoir to you.
Contact
 Direct contact: touching, kissing, sex, endogenous
spread (one part of you to another)
 Indirect contact, via fomites (inanimate objects)
 Droplet transmission:
less than 1 meter thru air
http://students.washington.edu/grant/random
/sneeze.jpg
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Transmission-2

Vehicles
 Water: various viruses, bacteria,
protozoa, mostly that cause diarrhea
and enter water supply.
 Food: unpasteurized or contaminated
food, either improperly grown,
processed, or prepared.
 Airborne: microbes attached to dust,
skin flakes, dried mucus become
aerosols, travel thru air.
http://www.kennethkeith.com/milkgreeceb.JPG
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Transmission-3

Vectors
 Typically arthropods (insects, ticks)
 Mechanical vectors: simply spread disease,
e.g. houseflies walking on feces, spread
germs to humans.
 Biological: pathogen goes through part of life
cycle in vector
 Viruses or protozoa that reproduce within
mosquito, e.g. Major method for spread of
zoonoses.
http://www.doktordoom.com/images/Tick.jpg
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Locally and Internationally
important diseases

The Commonplace
 Minor
respiratory diseases, i.e. common cold,
spread by contact.
 Digestive system: contaminated food, water;
unhygienic bathroom behavior and contact.

Regional

The highest incidence of tularemia, a
bacterial disease, is the Ozark Mts.
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
The Embarrassing: STDs
 Syphilis,
gonorrhoea, chlamydia all bacterial
diseases that can be cured with antibiotics.
 Herpes, genital warts, HIV are viral

Herpes is forever, wart virus causes cancer, HIV
causes death
 The
spread of STDs can be controlled by
change in behavior.
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
International
 HIV
is ravaging parts of the world, especially
Africa. Social, political, economic factors are
all involved.
 Malaria, caused by a protozoan, is still #1
cause of misery throughout the world.
 Lack of clean water, whether from poverty or
natural disaster, results in fecal-oral
transmission; bacterial diseases such as
typhoid fever, cholera, E. coli and viral
diseases like Hepatitis A.
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New and Emerging Diseases:
challenge for scientists
AIDS since 1970s
 Legionnaire’s disease since 1976
 Ebola just as recent
 West Nile spread to and thru US in last 5
years
 Ready for next flu pandemic? Is bird flu it?

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