Groups of organs working together to perform major activities of the body are called organ systems.

• The skeletal system, made of bones, cartilage and joints, is the framework of the body. It protects internal organs, stores minerals and provides a place for muscles to attach.
• The human muscular system is composed of smooth, cardiac and skeletal muscle tissue. Skeletal muscle, attached to the skeleton with dense strips of connective tissue called tendons, is responsible for the movement of body parts. Smooth muscle, sometimes called visceral muscle, is found in internal organs (eg, lines the walls of many blood vessels, makes up the iris of the eye and forms the wall of the gut). Cardiac muscle forms the bulk of the heart which controls blood circulation.
• The circulatory system-blood, blood vessels and the heart-is the body's transportation system, moving oxygen, carbon dioxide, nutrients, wastes, hormones, vitamins, minerals and water throughout the body. It also aids in regulation of temperature.
• The digestive system converts foods to simple substances that can be absorbed and used by the cells of the body. It is composed of the mouth, pharynx, esophagus, stomach, small intestine and large intestine and is aided by several accessory organs (liver, gall bladder, and pancreas).
• Made up of the skin, lungs, sweat glands and the kidneys, the excretory system removes metabolic wastes from the body. The kidneys are responsible for eliminating the bulk of wastes from the human body.
• The reproductive system generates reproductive cells (gametes) and provides a mechanism for them to be fertilized and maintained until the developing embryo can survive outside the body. The primary reproductive organs are the ovaries (female) and the testes (male).
• The nervous system regulates and coordinates the body's responses to changes in the internal and external environment. Major structures of the nervous system are the brain, spinal cord and nerves.
• The endocrine system consists of the hypothalamus, pituitary, thyroid, parathyroid and adrenal glands, as well as the pancreas, ovaries and testes. This system helps to maintain homeostasis, regulate temperature, and control growth, development, metabolism and reproduction by secreting and releasing hormones.
• The first line of defense in protecting the body is the integumentary system, which is composed of the skin, hair, nails, sweat and oil glands. It protects against injury, infection and fluid loss and also aids in temperature regulation.

A German physician named Robert Koch (winner of the Nobel Prize in 1905 for this work with tuberculosis) was the first scientist to connect a specific bacteria to a certain disease. He established the following four criteria for identifying pathogenic bacteria. These criteria are still the guidelines for medical microbiology today.

1. The pathogen must be found in an animal with the disease and not in a healthy animal.
2. The pathogen must be isolated and grown in a laboratory.
3. When the isolated pathogen is injected into a healthy animal, the animal must develop the disease.
4. The pathogen must be isolated in the second animal, grown in the laboratory and identified as the same pathogen.

In an experiment in 1796, Edward Jenner injected cowpox into healthy individuals in an attempt to cause each person to develop immunity to smallpox. These experiments led to the modern vaccination process. A vaccine is a solution containing dead or modified pathogens which is injected into a healthy individual to create immunity to a particular disease. Smallpox was the first disease to be considered eradicated worldwide by vaccination (1977).

Allergic responses are initiated when antigens (such as pollen, mold spores or dust) attach to mast cells causing them to release histamines. Histamines increase blood flow and secretion of fluids, prompting a range of symptoms. 

Autoimmune disorders occur when the body cannot distinguish pathogens from its own cells and tissues. Multiple sclerosis, rheumatoid arthritis and systemic lupus are examples of autoimmune disorders.
 
Acquired Immune Deficiency Syndrome, or AIDS, is an example of what can happen when the immune system becomes compromised. When the AIDS virus (HIV) becomes active, it causes a person to become sick from diseases that a functioning immune system would prevent.

The 19th century German philosopher, Ludwig Feuerbach, said, "A man is what he eats." Food fuels the body's cells and is used as building material for repair and in some cases, is stored for future use. The purpose of digestion is to break food down to molecules that can be used by cells. Digestion involves three principle processes: mechanical digestion, chemical digestion and absorption.

The process begins in the mouth. Chewing initiates the mechanical breakdown of food and is followed by secretion of saliva, which moistens and lubricates food for swallowing. Saliva also contains amylases (enzymes), which start the chemical breakdown of carbohydrates.
 
The swallowing reflex begins in the pharynx and initiates rhythmic waves of smooth muscle contractions called peristalsis. Peristaltic contractions transport food to the stomach and allow a person to swallow even if he/she are upside down.

The stomach contains an extra layer of muscle that aids in mechanically mixing and churning food into a semi liquid form called "chyme." Chemical digestion begins with proteins through the action of hydrochloric acid and the enzyme, pepsin. Only water and a few substances, such as aspirin and alcohol, are absorbed by the lining of the stomach.

As food enters the small intestine (so named because of diameter, not the length), secretions from the liver, gall bladder and pancreas are added. The small intestine completes digestion of food materials and the nutrients are absorbed into the blood. Fingerlike projections called villi (covered with microvilli) line the interior of the small intestine. Villi increase the surface area for absorption and secretion.

The large intestine is much shorter than the small intestine but is greater in diameter. The large intestine does not contain villi, which greatly reduces its available surface area for absorption. It has no role in digestion. Only water and vitamin K, synthesized by bacteria living in the the colon are absorbed into the bloodstream from the large intestine. Undigested or unabsorbed food is eliminated.
 
A nutrient is a substance the body needs for growth, repair and maintenance.  Nutrients include carbohydrates, proteins, lipids, vitamins, minerals and water.  Carbohydrates must be broken down into monosaccharides and are the body's main source of energy. Proteins are broken down to amino acids and supply the raw materials for growth and repair. The body requires 20 amino acids, 10 of which it cannot make and must obtain from outside sources. Lipids are reduced to fatty acids and glycerin. They are used to make steroid hormones, cell membranes and also store energy. Vitamins are organic molecules that aid in the regulation of body processes. Finally, water is required for metabolism and chemical reactions within the body, for transport of substances around the body, and for regulation of body temperature. Approximately two-thirds of the body weight is water.

Most disease-causing organisms, or pathogens, are too small to be seen without a microscope. Some (e.g., most viruses) are even too small to be visible under a light microscope and must be viewed with the more powerful electron microscope. Because of their microscopic size, these minute organisms often are referred to as microbes or microorganisms. The study of these organisms is called microbiology, and scientists who study these organisms are microbiologists. Not all microbes cause disease; many are beneficial and even essential. Bacteria, in the digestive system, for example are important partners in digestion. Microbes that cause disease are sometimes informally referred to as “germs” or “bugs”.

The five main groups of pathogens are bacteria, viruses, protozoa, fungi, and helminths. Bacteria are simple, single-celled organisms that lack an organized nucleus or membrane enclosed organelles. They often have a cell wall (prokaryotes), and their cells usually are rod-shaped or spherical. Commonly known diseases caused by bacteria are diarrheal diseases, pneumonia, strep throat, tuberculosis, and anthrax. 

Viruses are particles of nucleic acid (DNA or RNA) surrounded by a protective coat that replicate within specific host cells and can spread from cell to cell. Infectious diseases caused by viruses include the flu, the common cold, AIDS, chickenpox, and hepatitis. 

Protozoa are single-celled, motile, eukaryotic organisms, found in the Kingdom Protista, that can be human parasites. A protozoan known as Plasmodium (over 170 species), causes malaria, an infectious disease that is one of the world’s top killers.

Fungi are made of eukaryotic cells (organized nucleus and membrane enclosed organelles). All fungi, with the exception of the yeast group, are multi-cellular organisms that absorb nutrients from the environment. Fungi can cause athlete’s foot, sinusitis, skin diseases, and vaginal infections.

Helminths (worms and flukes) are invertebrate animals, some of which are parasitic. Wuchereia bancrofti is transmitted to humans by way of the mosquito. The mature adults pass into lymphatic glands, obstructing lymphatic drainage and resulting in a disfiguring condition, known as elephantiasis.

AIDS, caused by the virus, HIV, has become the most devastating infectious diseases of our lifetime. Since 1983, when HIV was first identified, approximately one million Americans have been infected. In 2006, approximately 40 million people were living with HIV infection worldwide, over four million became newly infected, and about three million died of AIDS. The epidemic is growing most rapidly among minority populations and women.

HIV infects white blood cells, primarily those called T cells, which are part of our immune systems. HIV destroys the ability of these cells to fight infection by other agents. Therefore, HIV-infected patients sometimes die from infection by other viruses, bacteria, or other agents that are not normally harmful to healthy individuals. Some AIDS patients also develop unusual cancers because of defects in their immune systems. HIV is not easily spread from person to person because it requires direct contact with the body fluids (blood and semen) of an HIV-infected person and it does not survive for very long in the environment. Transmission can occur through sexual contact, sharing of contaminated needles, transfusion of infected blood or blood products, and from mother to infant. There has not been documentation of HIV transmission in saliva or by blood carried by biting insects such as mosquitoes. 

There usually is a long lag period from the time someone is infected with HIV until the person begins to experience the symptoms of AIDS. (HIV infection does not follow the typical disease progression pattern described earlier, and the length of time to progression to AIDS can be highly variable.) Soon after infection, an individual may experience flu-like symptoms, but then remain asymptomatic (without symptoms) for up to a decade. However, during this phase, the virus continues to replicate and the infected person is contagious.

The steady increase in the number of HIV particles during the period while the virus is reproducing leads to a gradual decline in the level of immune system cells, known as CD4-positive (CD4+) T cells. A normal person has about 1,000 CD4+ T cells in a milliliter of blood. Once CD4+ T cell numbers fall to 200 cells per milliliter, the patient enters the phase of HIV infection known as AIDS. From this point on, it becomes increasingly difficult for patients to fight off infections. Signs and symptoms of HIV/AIDS include tiredness, fever, loss of weight, diarrhea, and swollen glands.

As of yet, there is no effective vaccine to prevent AIDS, and there is no cure. There are drugs that will reduce the number of HIV virus particles in the patient’s body (the viral load), which improves the length and quality of life. However, these drugs do not rid a person of HIV. Moreover, the drugs may become ineffective over time as the virus mutates and becomes resistant to the drugs. Once treatment is ceased, virus levels go back to earlier levels.

Currently, there are four classes of anti-HIV drugs that block three essential steps in the virus reproductive cycle: the entry phase (where the virus particle fuses with the host cell); the reverse transcription step (where the virus makes a DNA copy of its RNA genome); and the protease step (where a virus protein chops long strands of virus proteins into smaller, functional units). These usually are given in combination, as a “cocktail” called “highly active antiviral therapy,” or HAART for short. HAART is very effective for many people, at least for a period of time, but the treatment can produce unpleasant side effects and is too expensive for most people in the developing world. Most of all, it is important to remember that HAART is not a cure for AIDS.

There are many variations on the basic scheme of the virus life cycles described in the last two slides. As one example, we will choose HIV (human immunodeficiency virus), the virus that causes AIDS. HIV has a complicated life cycle that, while following the basic pattern, has some interesting and unique features. HIV is an enveloped RNA virus. It is a member of the retrovirus family. All members of this family contain RNA genomes that are converted into double-stranded DNA before new viruses can be made. These viruses have a “reversed” flow of genetic information and must convert RNA to DNA (hence the name retro) to reproduce. This process, is in contrast to the central dogma of molecular biology that states that information is passed from DNA, to RNA, to protein. Because cells do not have an enzyme that will generate new DNA from the existing virus RNA, retroviruses carry their own enzyme, called reverse transcriptase (shown in green), that makes double-stranded DNA copies from the information encoded in the RNA genome.

As with all viruses, HIV must first enter a cell before it can begin to reproduce. The HIV envelope glycoproteins bind to a specific receptor, called a CD4 molecule, found on the surface of white blood cells, usually T cells. The viral envelope fuses with the plasma membrane of the cell, and the capsid, containing two copies of viral RNA and two copies of the reverse transcriptase enzyme, enters the cell. The capsid proteins are removed (in a step called uncoating) and the reverse transcriptase begins the process of making a double-stranded DNA copy of the viral genome. The double-stranded viral DNA then travels from the cytoplasm into the nucleus of the cell, where it becomes integrated into the host cell DNA with the help of a viral protein called integrase. Viral DNA incorporated into the cellular DNA is called a provirus, analogous to the prophage of a lysogenic bacteriophage. In this state, it can be duplicated along with cellular DNA. Proviral genes are transcribed into RNA using the host cell transcription machinery. The RNA is then exported from the nucleus to the cytoplasm, where it serves as messenger RNA for the translation of viral proteins. The RNA molecules also function as genomes for the new viruses that are produced. The viral proteins and RNA assemble into viral particles and bud from the cell, acquiring an envelope in the process. The particles become mature, infectious viruses after undergoing an additional step involving a virus protein, referred to as protease, that cuts the long pieces of newly made virus proteins into smaller functional units.

Because the reproductive cycle of HIV relies on several virus proteins to produce new infectious viruses, steps requiring these proteins have been targeted in the design of drugs that interfere with virus replication. Current anti-HIV drugs block the entry, reverse transcription (AZT is one example of a drug that blocks this step), and protease steps. Research on drugs to block other parts of the HIV reproductive cycle is ongoing.

There are more than 20 types of known sexually transmitted diseases. These illnesses pass from one person to another, primarily through sexual contact, and can be caused by bacterial, parasitic or viral infection. The spread of many of STD-causing microorganisms can be prevented by the avoidance of sexual activity, or reduced by the proper use of latex (or polyurethane) condoms for all sexual contact. Prompt screening for and treatment of common STDs, such as chlamydia, also reduces the risk of spreading or contracting a more serious infection, such as by HIV.

HIV/AIDS refers to infection by the human immunodeficiency virus, which destroys the body’s ability to fight infections and cancers. HIV can be transmitted through any close contact with the body fluids, such as blood or semen, of anyone who has the virus. It also can be transmitted from HIV-infected mothers to their children. Transmission most frequently occurs during unprotected sexual acts or when sharing needles to inject intravenous drugs. An existing infection with another STD increases the risk of contracting HIV, because the HIV virus can invade the body through tiny sores caused by the other STD. Appropriate use of latex (or polyurethane) condoms reduces the risk of contracting or spreading HIV through sexual activity.

According to the National Institutes of Health, “there is no evidence that HIV is spread by contact with saliva or through casual contact, such as shaking hands or hugging, or sharing of food utensils, towels and bedding, swimming pools, telephones, or toilet seats. HIV is not spread by biting insects such as mosquitoes or bedbugs.”

The first signs of HIV infection generally occur a month or two after infection, and may be similar to symptoms of flu. Severe symptoms may not appear until many months or years later. More advanced stages of HIV infection—during which people are susceptible to many life-threatening diseases, including some forms of cancer—are referred to as acquired immunodeficiency syndrome (AIDS). Treatments now exist to fight HIV infection, but the virus always is deadly. HIV infection can be detected with a blood test.

There are more than 20 types of known sexually transmitted diseases. These illnesses pass from one person to another, primarily through sexual contact, and can be caused by bacterial, parasitic or viral infection. The spread of many of STD-causing microorganisms can be prevented by the avoidance of sexual activity, or reduced by the proper use of latex (or polyurethane) condoms for all sexual contact. Prompt screening for and treatment of common STDs, such as chlamydia, also reduces the risk of spreading or contracting a more serious infection, such as HIV.

Chlamydia, the most commonly reported bacterium-related STD in the US, is caused by the bacterium, Chlamydia trachomatis. An estimated 2.8 million Americans are infected with chlamydia each year, but many do not seek treatment because the symptoms often are mild or absent. The highest rates of infection are among adolescents between 15 and 19 years of age. Chlamydia can spread during any kind of sexual contact; the risk of infection can be reduced by correct use of latex (or polyurethane) condoms. Even though initial symptoms may be negligible, the infection is particularly dangerous to women, because it can damage a woman’s reproductive organs and cause serious problems, such as infertility, ectopic (outside the uterus) pregnancy and recurrent pelvic pain. Chlamydia can be treated and cured effectively with antibiotics.

Gonorrhea is an infection of the reproductive tract, mouth or anus caused by the bacterium, Neisseria gonorrhoeae. More than 700,000 persons in the US are infected with gonorrhea each year. Gonorrhea is spread through sexual contact. Several antibiotics can be used to cure this infection, but drug-resistant strains of gonorrhea are becoming more prevalent in many parts of the world. Correct use of latex (or polyurethane) condoms reduces the risk of contracting gonorrhea.

Herpes is caused by the Herpes simplex virus (HSV). Herpes infections around the mouth (cold sores) usually are caused by a strain of the virus called HSV type 1. Another variant of the virus, HSV 2 typically infects the genitals and anal area. HSV can be transmitted through any kind of sexual contact, including oral sex, and can spread even when sores are not present. Once a person is infected, the virus will remain in certain nerve cells of the body for life. There is no cure for herpes, but some antiviral medications can shorten or prevent outbreaks. Condom (latex or polyurethane) use reduces, but does not eliminate the risk of contracting HSV.

Human papillamoviruses (HPV) are common viruses that cause warts. About 30 types of HPV are transmitted sexually, and as many as 24 million Americans are infected. Some forms of HPV are linked specifically to the development of cervical cancers in women. Precancerous changes in the cervix can be detected by a Pap smear (a simple test performed in a doctor’s office). A new vaccine, Gardasil®, protects against four types of HPV that collectively cause 70% of HPV-related cervical cancers and 90% of genital warts. Correct use of latex (or polyurethane) condoms reduces, but does not eliminate, the risk of contracting or spreading HPV infection.

Trichomoniasis is caused by a single-celled parasite belonging to the protozoan group. It is the most common, curable STD in young, sexually active women. This disease also affects men, but the symptoms (irritation and yellowish discharge) are more common in women. Trichomoniasis usually can be cured with a prescription antibiotic. The use of latex condoms reduces, but does not eliminate, the risk of transmission.