Lesson Overview 35.1 Infectious Diseases

THINK ABOUT IT For thousands of years, people believed that diseases were caused by curses, evil spirits, or vapors rising from foul marshes or dead plants and animals. In fact, malaria was named after the Italian words mal aria, meaning “bad air.” This isn’t all that surprising, because, until microscopes were invented, most causes of disease were invisible to the human eye!

Causes of Infectious Disease What causes infectious disease?

Causes of Infectious Disease What causes infectious disease? Infectious diseases can be caused by viruses, bacteria, fungi, single-celled eukaryotes (formerly called protists), and parasites.

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.

Agents of Disease The single-celled eukaryote Plasmodium causes malaria, a very damaging infectious disease. The single-celled eukaryote Trypanosoma brucei feeds off nutrients in its host’s blood and causes African sleeping sickness. Both Plasmodium and Trypanosoma brucei are spread to human by insects. Giardia intestinalis causes infection of the digestive tract and is transmitted in infected water.

Agents of Disease People may be infected with the roundworm Trichinella spiralis from eating infected pork. The flatworm Schistosoma mansoni can be contracted by people working in rice paddies. Other parasitic worms include tapeworms and hookworms.

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.

Symbionts vs. Pathogens Some viruses and bacteria directly destroy the cells of their host. Other bacteria and single-celled parasites release poisons that kill the host’s cells or interfere with their normal functions. Parasitic worms may block blood flow through blood vessels or organs, take up the host’s nutrients, or disrupt other body functions.

How Diseases Spread How are infectious diseases spread?

How Diseases Spread How are infectious diseases spread? Some diseases are spread through coughing, sneezing, physical contact, or exchange of body fluids. Some diseases are spread through contaminated water or food. Still other diseases are spread to humans from infected animals.

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. If a virus infects only one host, that virus will die when the host’s immune system kills it or when the host dies. For that reason, natural selection favors pathogens with adaptations that help them spread from host to host.

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.

Coughing, Sneezing, and Physical Contact Those droplets also settle on objects such as doorknobs. If you touch those objects and then touch your mouth or nose, you can transfer the pathogens to a new home! The ability of a flu virus or a tuberculosis bacterium to cause a host to sneeze or cough is an adaptation that increases transmission of the pathogen from one host to another.

Coughing, Sneezing, and Physical Contact Other pathogens, including drug-resistant staphylococci that cause skin infections, can be transferred by almost any kind of body-to-body contact. They can also be transferred by contact with towels or certain kinds of sports equipment.

Coughing, Sneezing, and Physical Contact The most important means of infection control is thorough and frequent hand washing. If you have a cold or flu, cover your mouth with a tissue when you cough or sneeze, and wash your hands regularly.

Exchange of Body Fluids Some pathogens require specific kinds of direct contact to be transmitted from host to host. A wide range of diseases, including herpes, gonorrhea, syphilis, and chlamydia, are transmitted by sexual activity. Therefore, these diseases are called sexually transmitted diseases. Sexually transmitted diseases can only be completely prevented by avoiding sexual activity.

Exchange of Body Fluids 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, another adaptation that helps pathogens spread from one host to another, especially in places with poor sanitation.

Contaminated Water or Food Bacteria of several kinds are commonly present in seafood and uncooked meat, especially ground meat. If meats and seafood are not stored and cooked properly, illness can result.

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.

Zoonoses: The Animal Connection 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. Mosquitos can transfer West Nile virus between birds and humans.

Zoonoses: The Animal Connection In other cases, infection may occur when a person is bitten by an infected animal, consumes the meat of an infected animal, or comes in close contact with an infected animal’s wastes or secretions.