*Viruses and bacteria are present everywhere – in the air, on the ground, in the water, on inanimate objects, and even in and around your body.

-Some of these microorganisms are important in nature’s nutrient cycles and in the food and pharmaceutical industries, while others can cause diseases.

What is a virus?

  • Virus: a disease-causing, non-living particle
  • Does not grow, carry out respiration, or develop
  • The only thing it can do is replicate (make a copy of itself).
  • In order to replicate, it must enter a host cell.
  • Host cell: a living cell in wh/a virus replicates
  • Viruses are parasites and must remain inside living organisms in order to function!

Viral Structure

All viruses are made up of 2 main parts:

  • an inner core of nucleic acid, wh/contains the virus’s genetic material (either RNA or DNA)
  • an outer coat of protein, called a capsid

–          The arrangement of proteins in the capsid determines the virus’s shape:

–          Polyhedral (many-sided, crystal shape)

–          Long, narrow helical (cylinder)

–          Studded envelope

–          Complex: polyhedral head, helical

tail, leglike fibers

Attachment to a Host Cell

To replicate, a virus must enter a host cell.  To enter a host cell, viruses must recognize and attach to a receptor site on the plasma membrane of the host cell.

–          One of the virus’s proteins must interlock w/a molecular shape on the host cell’s plasma membrane, like two pieces of a jigsaw puzzle.

–          Attachment to a host cell is very specific– each virus has a specifically shaped attachment protein, therefore, each virus can only attach to a few kinds of cells.

o   Species specific/cell-type specific

o   Ex.- The polio virus only infects human intestinal and nerve cells; It can not infect any other type of cell or any other species of organism!

o   This is what makes controlling and eradicating certain viruses relatively simple.

o   Ex.- Smallpox was eradicated in 1980. How?

– Everyone was vaccinated, and the disease could not

spread to other organisms!

o   If a virus is NOT species specific, such as the flu virus, it is much

more difficult to control.

Viral Replication Cycles

*Once the virus is attached to the host cell’s plasma membrane, it releases its nucleic acid (DNA or RNA) into the host cell and takes over its metabolism!  Viruses can then replicate in one of two ways:

1. Lytic Cycle: type of viral replication in which the host cell is “taken over” and used to produce new viruses

–          Once new viruses are made, they burst from the host cell, killing it.

–          Each new virus can then infect and kill other host cells.  (See Fig.18.2,

2. Lysogenic Cycle: type of viral replication in whichthe virus’s nucleic acid becomes part of the host cell’s chromosome

–          provirus: viral DNA that is integrated into the host cell’s chromosome

–          The provirus is replicated along w/the host cell’s chromosome (every daughter cell will now have the provirus).

–          This replication can continue for many years, w/the provirus having no effect on the functioning of its host cell, however, at any time, the provirus can be activated and enter a lytic cycle- killing the host cell.

o   This explains the recurrence of cold sores, herpes outbreaks, etc.

o   What causes the provirus to become activated? Not sure.. possibly physical or emotional stress.


Retrovirus: an RNA virus, which must first make DNA from its RNA using an enzyme known as reverse transcriptase.

–          Since RNA is single-stranded, it can mutate much easier and faster,

making it harder to fight these viruses.

–          Ex.- the HIV virus

–          RNA → DNA → provirus in host chromosome

–          HIV infects white blood cells– an essential part of a human’s immune system!

–          HIV is a provirus, which means it is replicated along w/the host cells own DNA, and the infected cells are still able to function normally.

–          A person infected w/HIV may not show symptoms for years (provirus), however, they can still transmit this disease to others in their body fluids (Ex.- shared needles, sexual intercourse, blood transfusion).

–          Over time, more and more white blood cells are infected and you gradually lose white blood cells as they enter the lytic cycle.

–          ↓ # of white blood cells = ↓ immunity from diseases = AIDS (Acquired Immune Deficiency Syndrome)

–          Most people w/AIDS die of other diseases (pneumonia, the flu) due to their bodies weakened immune system being unable to defend itself against them.

*In summary: Scientists have identified thousands of viruses.  Some invade plants, others attack animals, and still others target bacteria.  A single drop of blood can contain 6 billion viruses.  Viruses lurk everywhere.

-We learned that bacteria are a type of prokaryote:  unicellular, living organisms that do not have membrane-bound organelles

–          Most bacteria are beneficial!!  Only a few cause serious illnesses, wh/we will discuss.

*Prokaryotes are classified into two kingdoms:

1. archaebacteria: “extremists”- live in extreme habitats w/little or no oxygen

–          Ex.- live in sewage plants; waters w/high salt content; acidic water along ocean floor vents; etc.

–          Thought to be the very 1st life forms on Earth!

2. eubacteria: live in more hospitable places

–          Ex.- heterotrophs (use other organisms as food); photosynthetic autotrophs (use sunlight to make food); chemosynthetic autotrophs (use inorganic substances to make food)

What Is a Bacterium?

Bacterium: small, single-celled prokaryote that has all of the structures necessary to carry out its life functions

Structure of Bacteria:

–          ribosomes for making proteins

–          genes located in a single, circular chromosome (plasmid)

–          plasma membrane for regulating what enters and leaves the cell

–          most have a flagellum that enable them to move

–          cell wall for support and protection

o   The cell wall is especially important as most bacteria live in a wet/hypotonic environment, in which water is always trying to enter the cell by osmosis.

o   W/o the cell wall, water would fill the bacteria cell, causing the cell to burst!

o   So… if you want to kill a bacteria cell, simply damage its cell wall, which will cause the cell to fill w/ water and rupture and die.

o   This is how penicillin works!

§  Penicillin: the 1st antibiotic used in humans; causes bacteria to develop w/holes in their cell walls, thus allowing water to enter the cell, thus causing the cell to burst and die.

Identifying Bacteria

3 Main Shapes of Bacteria:

1. spheres (cocci)

2. rods (bacilli)

3. spirals (spirilla)


*Bacteria are prokaryotes, so they have no nucleus, which means they cannot undergo mitosis or meiosis.

–          Instead, they reproduce asexually in a process called binary fission: when one cell divides into two separate genetically identical cells  How??

o   The cell copies its chromosome (now has 2), the chromosomes move to opposite ends of the cell, the cell grows and pinches off between these two separated chromosomes.

o   Happens very rapidly!

o   Some species can reproduce every 20 minutes!!

–          Some bacteria have a form of sexual reproduction called conjugation: when

one cell transfers all or part of its chromosome to another cell using a

bridgelike structure called a pilus (plural, pili), that connects two cells.

– results in bacteria w/new genetic makeup!

Adaptations in Bacteria

*All bacteria cells are living, which means they carry out cellular respiration: breaking down food to release and use its energy.  Modern bacteria have 2 main types of respiration:

Obligate aerobes: bacteria that require O2 for respiration

Obligate anaerobes: bacteria that are killed by O2 and thus do NOT require O2 for respiration

*When faced w/unfavorable conditions, some bacteria can put their respiration on hold and enter a state of hibernation or rest.

–          endospore: a structure formed by bacteria during unfavorable conditions

–          a tough, protective, outer covering that resists drying out, temp. extremes, and harsh chemicals

–          can exist for thousands of years

–          when environment improves, the bacteria cell begins to grow and reproduce again

–          Endospores can pose big problems to people, as they are very tough to kill!!

–          This is why many items are sterilized, acidified, etc.- To kill the tough endospores of bacteria!

The Importance of Bacteria

*As stated earlier, most bacteria are actually beneficial!

  • nitrogen fixation: convert N in the air into N-compounds that plants can use (nitrogen fixation); we then eat these plants to get the N we need!
  • recycle nutrients on Earth: break down dead/decaying organisms and return these nutrients to the environment (essential in all food chains)
  • produce foods: many bacteria have distinctive flavors and aromas, making them good in food

Ex.- swiss cheese, pickles, yogurt, vinegar, sauerkraut

  • produce medicines: some bacteria produce antibiotics that kill other (harmful) types of bacteria

*Bacteria also cause about ½ of all human diseases.  They can enter our bodies through openings, such as the mouth or a skin wound, and are carried in air, food, soil, and water.  Improvements in medicine and public health have reduced the death rates from bacterial diseases; however, we still have a lot to learn about treating and curing many of the illnesses caused by bacteria.

The Nature of Disease

*Your body constantly faces attack from disease-causing organisms.

–          Disease: a change that disrupts the homeostasis in the body

–          Pathogens: disease-producing agents such as bacteria, viruses, fungi, and other parasites

–          Infectious disease: caused by the presence of pathogens in the body; are usually contagious

Ex.– chicken pox, colds, flu, HIV, rabies

–          Noninfectious disease: caused by genetics, aging, or other ways NOT related to pathogens; are usually NOT contagious

Ex.– cancers, cardiovascular disease, hemophilia

The Spread of Infectious Diseases

*The main cause of human disease pathogens is the human body itself!

–          The major nonliving reservoirs of infectious diseases are soil and water, which are often filled with disease-causing microorganisms and/or contaminated by the feces of other organisms.

–          Pathogens are transmitted from reservoirs to humans in 4 main ways:

  • direct contact– touching, kissing, etc.
  • by an object– door handle, drinking glass, etc.
  • through the air– on droplets of water or dust that you then breathe in
  • by a vector– an intermediate organism, such as an insect, that transmits disease

Ex.- a fly lands on a contaminated object (poo) and then lands on your food

Ex.- a tick gets Lyme disease from a dear and then bites you and gives you the disease

What causes the symptoms of a disease?

–          Viruses: take over host cell’s metabolic activity, eventually causing them to burst and die (kills cells)

–          Bacteria: damages/kills host cells by producing toxins: powerful and deadly poisons produced by many types of bacteria

o   Toxins are transported by the blood throughout the body, causing serious and sometime fatal side effects (fever, muscle spasms, death to blood cells, internal bleeding, nausea, vomiting, etc.)

Patterns of Diseases

*In today’s highly mobile world, diseases can spread rapidly.

CDC (Center for Disease Control): the central source of disease information in the U.S.

o   contains up-to-date info. on incidences of diseases and outbreaks in the U.S. and around the world, as well as info. on required vaccinations for travel, etc.

–          Endemic disease: a disease that is constantly present in the population  (Ex.- the common cold)

–          Epidemic: occurs when many people in a given area are afflicted w/the same disease at about the same time  (Ex.- the flu is often an epidemic)

Treating Diseases

*There are many options when it comes to treating diseases today, but most commonly used are medicinal drugs, such as antibiotics.

Antibiotic: a substance produced by microorganisms that will kill or inhibit the growth/reproduction of other microorganisms, especially bacteria (usually work by destroying the cell wall of the bacteria cell).

–          Antibiotics can be used to cure some bacterial infections, but they DO NOT AFFECT VIRUSES!!

–          Another problem w/antibiotics: ↑ use of antibiotics is causing more and more types of bacteria to evolve in ways that make them resistant to it!  As bacteria continue to evolve, we must continue to find new ways to destroy it.

–          Antibiotics are only 1 way to fight infections: The main way we fight infections is by using our own, natural, built-in defense system- the immune system.

Defense Against Infectious Diseases

*An immune system is a collection of mechanisms within an organism that protects against disease by identifying and killing pathogens and tumor cells.

–          It detects a wide variety of agents, from viruses to parasitic worms, and needs to distinguish them from the organism’s own healthy cells and tissues in order to function properly.

–          Our bodies have many different types of immunity for fighting disease:

Innate immunity: (nonspecific immunity): your body’s earliest lines of defense against any and all pathogens

o   Skin: your body’s 1st line of defense!

§  The largest organ in your body.

§  A formidable physical barrier between the outside environment and our inner bodies.

§  The 1st barrier a pathogen must penetrate in order to enter your body and cause disease.

§  If the skin is damaged (ex.- burn, cut, etc.), pathogens can enter our systems.

o   Body secretions: your body’s 2nd line of defense

§  Ex.- mucus, sweat, tears, saliva, stomach acid

§  Defend our bodies in 1 of 2 ways:

  • Since they are wet/sticky, they can trap microorganisms that try to enter our bodies before they get all the way in.

– Ex.- We then sneeze, cough, spit them out

  • They also contain enzymes which are capable of breaking down the cell walls of some bacteria, rendering them harmless.

o   Inflammation: redness, swelling, pain, and heat that occurs in body tissues that have been damaged by pathogens or some form of physical injury

§  Due to the onslaught of white blood cells and tissue fluid that rushes to the area to help the body destroy toxic agents and restore homeostasis.

§  Phagocytes: white blood cells that destroy pathogens by surrounding and engulfing them

  • types include: macrophages, neutrophils, monocytes, and eosinophils, which are all present in various areas of our body and defend us from many different things, however…
  • Phagocytes can NOT destroy viruses!!
  • Since viruses multiply within a host cell, a phagocyte that destroys a virus will destroy itself if the virus multiplies within it!!

o   Protective proteins (Interferons): proteins that protect cells from viruses by preventing the viruses from multiplying

§  Host-cell/species specific

§  Produced by a body cell that has been infected by the virus

Acquired immunity: (specific immunity): your body’s defense against a specific pathogen/antigen by gradually building up resistance/antibodies to it

o   Antigens: foreign substances that stimulate an immune response

§  Normally, your body recognizes your own cells as ‘self’ and foreign cells (antigens) as ‘nonself’.

o   Antibodies: proteins that our bodies produce in response to the presence of antigens

§  Attack and kill invading antigens.

§  Correspond specifically to each antigen.

o   Acquired immunity develops naturally over time as you are exposed to more and more antigens, which your white blood cells are gradually able to recognize and destroy more swiftly.

o   Once exposed to a specific antigen, the antibodies that destroyed that antigen stay in your blood stream in the form of memory cells.  These memory cells are armed and ready to respond rapidly if the same antigen invades the body at a later time; this response to a second invasion is immediate, usually w/o any symptoms.

o   This explains why children get sick more often than adults- their acquired immunity is still developing!

o   This also explains why you can’t get the same disease twice!  (Ex.- chicken pox, measles, mumps)

o   The development of acquired immunity is the job of the lymphatic system, which includes:

o   The process of acquiring immunity to a specific disease can take days or weeks.

You may develop immunity to a specific pathogen in one of two ways

  • Passive Immunity: immunity that results from receiving antibodies from another person who is already immune to the disease

Ex.- through injection; through mother’s breast milk

  • Active Immunity: immunity that results from being exposed to the antigen and building up your own antibodies to the disease

–          Can be developed in two ways:

a. getting the disease and becoming ill from a specific pathogen and subsequently destroying it

b. receiving a vaccine: a weak or dead form of a specific antigen that is injected into your body, thus causing an immune response

–          Since the antigen is a weak or dead form, our bodies can quickly and easily destroy it.

–          Our bodies are left with antibodies that can then combat the antigen if we were to ever come in contact with it again!

–          This virus is so deadly because it breaks down the body’s immune system, which makes us unable to defend ourselves against antigens.

–          It is also a provirus, which means it incorporates it’s DNA into our own cell’s DNA, and can remain inactive and symptom-free for 8-10 years!  This entire time, however, you could unknowingly pass this virus on to others.

–          People who eventually develop AIDS almost always die of normally non-fatal infectious diseases, which take advantage of the body’s weakened immune system.

Immune System Disorders

–          Allergies: the result of our immune system initiating an inflammatory response to substances that are normally NOT harmful

o   Why? Many allergies

are inherited (genetic)

o   People may be allergic to:

Autoimmune diseases:

Result when a person’s immune system fails to distinguish ‘self’ from ‘nonself’, and attacks their body’s own healthy cells as if they were antigens

o   Ex. – Rheumatoid Arthritis– inflammation in joints

–          Multiple Sclerosis– your body attacks the lining of your nerve cells

–          Lupus– your body attacks your tissue organs and joints

*Most of the time, our bodies immune system functions properly, enabling us to face the constant barrage of unseen enemies (mainly, viruses and bacteria) without fear.

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1 Comment on "Viruses and Bacteria & Immunity from Disease"

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Lois Bennin

September 29, 2011
Dear Phil,
I enjoyed using your article “Viruses and Bacteria & Immunity from Disease” while tutoring a student preparing for an online certification exam. I knew you would want to know that, near the end of the article just prior to the section on “Immune System Disorders”, there appears to be a missing passage that would identify the three points as information about HIV/AIDS.
I look forward to using some of your other articles in the future.
Lois Bennin, retired teacher and tutor, Beverly Hills, Florida