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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 microbesBacteria, viruses, a few fungi, and a few

protistsWhat 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 becauseThey are not made of cells.They cannot reproduce without a host cell.Plus other excuses.

Not made of cellsThis 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 capsidA 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 nmThe 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/.../ Figures/Icos_Virus.GIF; http://www.foresight.org/Updates/Update48/Images/T4Schematic.jpg

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Examples of virus shapes

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

Ebola Adenovirus

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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 cellsAttachment 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 limits re using humans Eggs

In 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 energyViruses break open host cells when they

multiplyCells die

Your immune system kills infected cellsWhite 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 cancerKaposi’s sarcoma virus: cancer with AIDSPapilloma virus: genital warts and cervical

cancerEpstein-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 reservoirCould 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: STDsSyphilis, 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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InternationalHIV 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?