Why is the Immune System Important Why is the Immune System Important? Do Now: What are the different ways you can get sick? Workbook pages due tomorrow

Causes of Infectious Disease During the mid-nineteenth century, Louis Pasteur and Robert Koch established a scientific explanation for infectious disease. Pasteur’s and Koch’s observations and experiments led them to conclude that infectious diseases are caused by microorganisms. Microorganisms were commonly called “germs,” so this conclusion was called the germ theory of disease.

Agents of Disease Infectious diseases are caused by pathogens— organisms that invade the body and disrupt its normal functions. Examples of pathogens are viruses, bacteria, single-celled eukaryotes, fungi, and parasites.

Agents of Disease Viruses are nonliving particles that replicate by Inserting their genetic material into a host cell and taking over many of the host cell’s functions. Viruses cause the common cold, influenza, chicken pox, and warts.

Agents of Disease Bacteria cause disease by breaking down the tissues of an infected organism for food, or by releasing toxins that interfere with normal activity in the host. Bacteria cause streptococcus infections, diphtheria, botulism, and anthrax.

Agents of Disease Different types of fungus may infect the surface of the skin, mouth, throat, fingernails and toenails. Dangerous infections may spread from the lungs to other organs. The fungus Trichophyton interdigitale causes athlete’s foot.

Koch’s Postulates Koch’s studies with bacteria led him to develop rules for identifying the microorganism that causes a specific disease. These rules are known as Koch’s postulates. The pathogen must always be found in the body of a sick organism and should not be found in a healthy one. The pathogen must be isolated and grown in the laboratory in pure culture.

Koch’s Postulates Koch’s studies with bacteria led him to develop rules for identifying the microorganism that causes a specific disease. These rules are known as Koch’s postulates. 3. When the cultured pathogens are introduced into a healthy host, they should cause the same disease that infected the original host. 4. The injected pathogen must be isolated from the second host. It should be identical to the original pathogen. Although there are exceptions to these rules, they remain important guidelines for identifying the causes of new and emerging diseases.

Symbionts vs. Pathogens Most microorganisms that live and grow in the human body are symbionts that are either harmless or actually beneficial. Yeast and bacteria grow in the mouth and throat without causing trouble. Bacteria in the large intestine help with digestion and produce vitamins.

Symbionts vs. Pathogens What’s the difference between harmless microorganisms and pathogens that cause disease? The “good guys” obtain nutrients, grow, and reproduce without disturbing normal body functions. The “bad guys” cause problems in various ways. Some viruses and bacteria directly destroy the cells of their host.

How Diseases Spread Pathogens are often spread by symptoms of disease, such as sneezing, coughing, or diarrhea. In many cases, these symptoms are changes in host behavior that help pathogens spread and infect new hosts.

Coughing, Sneezing, and Physical Contact Many bacteria and viruses that infect the nose, throat, or respiratory tract are spread by indirect contact. Coughing and sneezing releases thousands of tiny droplets that can be inhaled by other people. The most important means of infection control is thorough and frequent hand washing.

Exchange of Body Fluids Some pathogens require specific kinds of direct contact to be transmitted from host to host. Other diseases, including certain forms of hepatitis, can be transmitted among users of injected drugs through blood from shared syringes. HIV can be transmitted through blood or sexual contact.

Contaminated Water or Food Many pathogens that infect the digestive tract are spread through water contaminated with feces from infected people or other animals. Contaminated water may be consumed, or it may carry pathogens onto fruits or vegetables. If those foods are eaten without being washed thoroughly, infection can result. Symptoms of these diseases often include serious diarrhea

Zoonoses: The Animal Connection Any disease that can be transmitted from animals to humans is called a zoonosis. Mad cow disease, severe acute respiratory syndrome (SARS), West Nile virus, Lyme disease, Ebola, and bird flu are all zoonoses. Sometimes an animal carries, or transfers, zoonotic diseases from an animal host to a human host. These carriers, called vectors, transport the pathogen but usually do not get sick themselves.