The human body is constantly affected by organisms such as bacteria, viruses, fungi, and protozoa.  Some are beneficial such as the intestinal bacteria that produce some of our vitamins.  Others are harmful causing illnesses that range from mild colds to life threatening meningitis.  The immune systems produces antibodies that fight infections some of which help prevent reinfection.  


Bacteria that are present in soil, water, and air can cause serious illness such as tetanus, typhoid, and pneumonia.  Fortunately antibiotics are effective against most bacteria, some work by destroying the bacterial cells wall.  In addition to antibiotics some vaccines can prevent certain bacterial infections such as tetanus and diphtheria.  There are single celled organisms whose shapes vary greatly.  In general, bacteria can be divided into three general categories based on shape: cocci (round), bacilli (rod shaped), and spirella (spiral shaped).  


Some bacteria adhere to and invade tissue cells such as the dysentery causing Shigella.  Others produce poisonous substances known as toxins.  Some toxins are very dangerous: 7 pounds of botulism could kill all the people in the world.  Toxins may alter certain chemical reactions in cells so that normal cells function is disrupted or the cell dies.  One example is the diphtheria toxin, which damages heart muscle by inhibiting protein synthesis. Some toxins cause blood to clot in small blood vessels.  Areas of tissue supplied by these vessels may be deprived of blood and damage the cell walls of tiny blood vessels, and thereby cause leakage.  The fluid loss results in decreased blood pressure, and the heart is unable to supply adequate amounts of blood to the vital organs.  Since the introduction of penicillin in 1940, many bacteria have developed intricate ways of resisting antibiotics.  The most effective mechanisms are the rapid transfer of plasmids, small packages of the cell's genetic material - DNA, between bacteria and populations that contain resistant genes.  Bacteria that receive these plasmids also inherit the resistant genes, and therefore develop the same resistance that the donor bacterium possessed.  

Plasmids signal bacteria to produce enzymes that can inactivate drugs.  They may also stimulate a bacterium to alter its receptor site where antibiotics normally bind.  Plasmid transfer takes place during the process of conjugation.  The plasmid duplicates itself in a donor bacterium.  This copy passes through a tube to a pilus to the recipient cell.  Plasmid transfer may produce large populations of bacteria that form enzymes against a range of antibiotics.  

E. coli


Infection from viruses cause a wide range of diseases including colds, hepatitis, and AIDS.  A virus cannot be killed by antibiotics; the body must produce specific antibodies to combat each virus.  However, newer anti-viral drugs will control some viruses.  Each virus has a core of nucleic acid composed of DNA or RNA and one or two protein shells.  Surface proteins, or antigens, stud the outer shell. 

Because viruses have few genes, they are unable to process nutrients, nor can they reproduce independently.  All viruses must invade host cells which die or function abnormally.  Some viruses provoke the immune system to destroy the body's own normal cells.  

Before a virus invades a host cell, it’s surface proteins must attach to specific receptor sites on the surface.  After attaching itself, part or all of the virus penetrates the host cell and sheds its protein coat to release it’s nucleic acid.   

The nucleic acid makes copies of itself, using the hosts cells raw materials, and sometimes its enzymes to do so. Replicated nucleic acid generates new virus particles.  

The cell swells with new virus particles and may burst and die.  When the cell bursts, the virus particles are released; these subsequently infect other cells.  However, not all viruses destroy cells as they leave.  Instead, some, such as herpesviruses form buds and take away a part of the host cells membrane with them. They are known as enveloped viruses.

The three types of influenza viruses are A, B, and C.  Each of these changes its structure periodically.  The antibodies in a vaccine that defended against a previous viral structure may be infective against a new one so that reinfection occurs. This alteration in structure, called the antigenic shift, occurs in the surface proteins (antigens) where antibodies attach.  

ebola virus

Depending on the main constituent of the genetic material, viruses can be broadly classified as RNA viruses or DNA viruses.  Further classification is governed by the size, shape, and symmetry of the virus.  The most common families include:

Adenoviruses: DNA viruses. Infections of the tonsils, respiratory tract, and the eyes (such as conjunctivitis). 

Papovaviruses: DNA viruses.  Initiate benign or non-cancerous tumors such as warts on the hands and feet. 

Herpesviruses: DNA viruses.  Cause cold sores, genital herpes, chicken pox, shingles, and glandular fever.  



Coronaviruses: RNA viruses.  Named for their resemblance to the sun, they cause the common cold.

Picornaviruses: RNA Viruses.  Cause myocarditis, polio, viral hepatitis, and one form of meningitis.  

Retroviruses: RNA viruses that can convert RNA into DNA.  They cause AIDS and a type of leukemia.   


Reoviruses: RNA viruses. Cause respiratory infections; a group (rotaviruses) cause gastroenteritis.   

Orthomyxoviruses: RNA viruses.  Cause influenza, symptoms, which include fever, cough, and a sore throat.

Paramyxoviruses: RNA viruses.  Causes mumps, measles, rubella, and respiratory infections such croup.  

How do you tell whether you have a cold or the flu? The following chart will aid you in making a differential diagnosis:



Tired & Weakness
Chest Discomfort
Stuffy Nose
Sore Throat




101+(3-4 days)
usually severe
2-3 weeks
early & prominent



Protozoa are single celled primitive animals, some of which are parasites that can cause diseases in humans. Malaria and toxoplasmosis are caused by protozoa parasites, which are particularly common, affecting over a third of the world’s population.  Parasites employ various mechanisms for evading the immune system.  The Leishmania parasite, responsible for causing a disease called kala-azar, lives and multiplies within phagocytes, defense cells forming part of the immune system.

Protozoa have a large, well-defined nucleus and no cell wall. Many, such as Trypanosoma have one or more tail-like appendages known as flagella, which aid movement. Amoebas, such as Entamoeba histolytica can ingest red blood cells and food particles.  

Four types of Plasmodia can cause malaria.  Spread by the bite of the female Anopheles mosquito, most of these have a similar life cycle and produce chills and fever. one type, Plasmodium falciparum affects vital organs such as the kidneys and brain, and can be fatal within hours if not treated immediately.  Because resistance to anti-malarial drugs is increasing, scientists are trying to develop vaccines.

If the female Anopheles mosquito bites, injecting saliva that contains sporozoites, the infected form of the malaria parasite, sporozoites enter the liver cells and multiple.  The sporozoites develop into merozoites, another form of the parasite.  Merozoites are released from the liver cells into the blood stream and then invade red blood cells where they multiple.  Red blood cells then rupture and release the merozoites which invade other RBC's and cause recurring chills and fever.  Some parasites develop into gametocytes, male and female cells that reproduce if ingested by a mosquito.


Fungi are simple organisms that scavenge dead or rotting tissue.  Some fungi infect humans, causing either harmless superficial diseases of the skin, hair, nails, or mucous membranes, or possibly fatal infections of certain vital organs, such as the lungs.  

Cryptococcosis is an infection that causes pneumonia, meningitis and can affect the skin and bones.  Aspergillosis may spread through either ventilation systems or humidifiers to infect the lungs.  Dermatophytosis is a skin infection, also called tinea, most commonly affects the scalp, feet, and nails. Candidiasis may infect the mouth and genitals, and may occur in the heart, bowel, bladder, and brain. Histoplasmosis is associated with soil that is contaminated by bird droppings.  Fungal spores are inhaled by humans and can cause pneumonia.  The fungus can spread to and infect organs through out the body.   


candidiasis cells

Fungal infections respond in a variety of ways to antifungal drugs.  Superficial infections like thrush (oral candidiasis) respond to the local applications of antifungal drugs.  Deep infections in people with lowered immunity are difficult to cure, and often require prolonged therapy with more toxic drugs. 

Fungal cells, which are harder to treat than bacteria, closely resemble human cells.  Drugs must affect fungal cells but not cause excess damage to body cells in the process.  Most antifungal drugs form channels in the membrane of the fungal cell through which vital cell contents can leak out, causing cell death.  


Diagnosis of infectious diseases is usually confirmed in a laboratory by techniques such as microscopy and culture.  Many bacteria are colorless and consequently require special staining procedures.  One commonly used stain is Gram's stain. 

Certain bacteria or fungi are identified when specimens are cultured by growing them on plates until colonies are visible.  Viruses are cultured on live cells or eggs. 

To find the best treatment, bacterial colonies may be transferred onto plates with disks that are saturated with different antibiotics. No bacteria grow around the most effective antibiotic.

Viruses are too small to be seen with a light microscope.  Instead, infections caused by viruses are diagnosed indirectly by their effects on cells.  Some viruses alter the surfaces of cultured cells causing them to agglutinate, or stick together.   

A single layer of tissue cells is cultured on a prepared plate.  A specimen, such as blood from an infected person, is added to the plate.  Presence of the virus can be identified in several ways. 


Some infectiousness diseases are common and can reoccur in the same person.  Others occur only once in a lifetime because the immune system can remember the organism and resist subsequent infections.  To prevent an epidemic of serious infectious diseases, such as polio, immunization artificially creates a “memory” before the disease can be acquired.  

With active immunization, a vaccine with dead or harmless living forms of an organism is injected into a person.  The vaccine stimulates the immune system to memorize the organism and produce antibodies.  In any subsequent infection with this organism, the antibodies stop the infection. 

With passive immunization, blood with antibodies is taken from animals or humans who have had the infection recently.  Blood serum containing antibodies is separated from the blood, processed, and injected.  Antibodies either attack a current infection or provide short-term protection.

Genetic engineering is a tem that describes a technique that alters the genetic material (DNA) of an organism by inserting the genes from another organism. Viral genes are inserted into the DNA of other organisms.  When these organisms multiply, the large amounts of replicated material are used as vaccines or hormones.   

With the hepatitis B vaccine, the gene of the surface antigen (protein) of the hepatitis B virus is inserted into a bacterium’s DNA.  The cell produces viral antigens, which are injected to stimulate an immune response.  

Common Immune Disorders 

In allergies and autoimmune diseases, the immune system overreacts; in immunodeficiency diseases, the defense systems are too weak to cope with threats to health.  

Allergy is an inappropriate response by the immune system to a substance that, for most people, is harmless.  These substances, known as allergens, may be inhaled or swallowed, or they come into direct contact with the eyes or skin.  The allergic responses, such as hay fever, asthma or rashes.  Some people are allergic to eggs, milk, and grains. 

Allergies can provoke the immune system to produce an antibody called immunoglobulin E (IgE).  The IgE molecules coat the surface of mass cells, which are located in the skin and in the lining of the stomach walls, lungs, and upper airways.   

In an allergic person, subsequent exposure to allergens, which may be ingested or inhaled, causes them to bind to the IgE molecules. This is known as cross-linking.

Cross-linking causes granules located inside the mast cell to release the inflammatory substances histamine and prostaglandins, which trigger types of allergic response.

Autoimmune Disorders

Sometimes theimmune system forms antibodies not against invaders such as bacteria, but against some of the bodies own tissues.  The mistaken attack may be directed against a particular organ, such as the thyroid gland, or it may cause a more generalized illness.  These diseases become more common in middle age and affect women more often than men.

Vitiligo: Cells called melanocytes produce a skin darkening pigment called melanin.  The multiple irregular areas of depigmentation are treated with a corticosteriod drugs. 

Addison’s Disease: Damaged adrenal gland cortexes cause low blood pressure and weakness, lowering the body’s ability to respond to stress. 

Diabetes mellitis (insulin dependent): Clusters of pancreatic cells known as islets of Langerhans  are destroyed, and are unable to produce sufficient insulin for glucose control.

Pernicious anemia: Results when the stomach lining cells are attacked and destroyed by antibodies formed elsewhere in the body.   

Graves' disease: The thyroid becomes overact and may become enlarged, forming a goiter.  There is loss of weight, restlessness, and tremor.  

Multiple sclerosis: Damage to the nerve fiber coverings causes muscle weakness, disordered sensations, and problems with speech and vision.  

Myasthenia gravis: Damage to the nerve receptors of the muscles cause muscle weakness, which is especially noticeable  in the muscles of the face.   

Systemic lupus erythematosis: Multi-organ damage results in progressive loss of function in the kidneys, lungs, and joints. A typical rash appears over the cheekbone (see Dermatology lesson).


Acquired immunodeficiency syndrome, or AIDS, is caused by the human immunodeficiency virus (HIV).  The virus destroys one type of white blood cell, the CD4 lymphocyte.  As the number cells of declines, the immune system becomes less effective and death may occur approximately 10 years after infection.  HIV is spread by sexual intercourse and contaminated blood. 

T-cell undergoing apotosis

HIV is a spherical virus with several protein coats.  Its genetic material is composed mainly of RNA and enables the virus to multiple inside infected cells.  The antigens on the outside envelope allow the virus to attach to and infect CD4 lymphocytes.  

Many infected people have no symptoms for many years and are known as “asymptomatic carriers.”  In later stages, they lose weight, develop night sweats, fever, and diarrhea.  In full-blown AIDS, infected people become susceptible to a wide variety of infections and to types of cancer.  

Someone whose immune system has been damaged by HIV often develops lung infections.  One type commonly associated with this Pneumocystis carinii pneumonis, or PCP.

Kaposi’s sarcoma is the cancer that is often associated with AIDS.  Brown or blue patches and nodules appear on the skin.  These gradually spread to all parts of the body surface and also affect internal organs.  

Persistent diarrhea is one of the most common features of AIDS.  It is caused by the infection of the gastrointestinal tract with parasites, for example Giardia and Cryptosporidium and with fungi, especially Candida.  

If HIV infection spreads to the brain and nervous system is causes mental disturbances.  Blindness may result from infection with cytomegalovirus. 

A blood test for HIV infection looks for the antibodies to the virus, which are easy to detect.  The technique employed is the enzyme-linked immunosorbent assay, or ELISA.  If antibodies are found, then a confirmatory test called the Western Blot is done.  If both test results are positive, the person is HIV positive.

Antigens, or surface proteins, from the AIDS virus are first spread on a prepared test surface or on the inside of a test tube.  The test surface is exposed to blood serum.  If any HIV antibodies are present, they will bind to the HIV antigens. The test surface is washed.  A special chemical linked to the enzyme peroxidase, which is known to bind to HIV antibodies, is added to the test surface.  The surface is then washed again.  A reagent, a substance that is used to analyze biological substances, is added to the test surface.  Any HIV antibodies will change the color of the reagent, producing a positive test result.   

Review Questions:

1. What is the difference betwen antibodies and antibiotics?

2. How are bacteria and viruses different?

3. How do bacteria develop resistance to antibiotics?

4. Why don't vaccines for influenza work very well?

5. How can you tell whether you have the flu or a cold?

6. How are fungi and bacteria different?

7. Why is it so difficult to treat fungi?

8. What's the difference between allergies and autoimmune diseases?

9. Describe what AIDS is and how it is transmitted. Does a person who is HIV positive have AIDS? Why or why not?

10. Describe how diagnosis is made of immune diseases?