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 1900s, Louis Pasteur & Robert Koch established a scientific explanation for disease. Pasteur’s & Koch’s experiments led them to conclude that infectious diseases are caused by microorganisms. Microorganisms were commonly called “germs” -germ theory of disease.

Agents of Disease pathogens—organisms that invade the body and disrupt its normal functions. Ex.) 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 ex,) common cold, influenza, chicken pox, and warts. Bacteria cause disease by breaking down the tissues or by releasing toxins ex.) streptococcus, diphtheria, botulism & anthrax. Fungus may infect the surface of the skin, mouth, throat, fingernails and toenails. ex.) athlete’s foot & ringworm

Agents of Disease The single-celled eukaryote Plasmodium causes malaria. The single-celled eukaryote Trypanosoma brucei causes African sleeping sickness. Both 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 from eating infected pork. The flatworm 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. Although there are exceptions to these rules, they remain important guidelines for identifying the causes of new and emerging diseases.

Symbionts vs. Pathogens Symbionts=Good Guys Yeast and bacteria grow in the mouth and throat without causing trouble. Bacteria in the large intestine help with digestion and produce vitamins. Pathogens=Bad guys 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

Symbionts vs. Pathogens

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.

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. (vectors)