Understanding Infectious Diseases: Causes, Spread, and Immune Responses

Infectious Diseases: An Overview

Changes to body physiology that disrupt normal body functions and are caused by microorganisms are called infectious diseases. This explanation, established by Louis Pasteur and Robert Koch, is called the germ theory of disease.

Causes of Infectious Diseases

Infectious diseases are caused by various microorganisms, including:

• Viruses
• Bacteria
• Fungi
• Protists
• Parasitic worms

Disease-causing microorganisms are also called pathogens. Koch also developed a series of rules, known as Koch's postulates, that help scientists identify which organism causes a specific disease.

While many microorganisms are symbionts that are either harmless or beneficial, pathogens cause disease by:

• Destroying cells
• Disrupting body functions
• Releasing toxins that kill cells or interfere with their normal functions

How Diseases Spread

Infectious diseases can be spread in several ways:

Person-to-Person Transmission

Some infectious diseases are spread from person to person through:

• Coughing
• Sneezing
• Physical contact
• Exchange of body fluids

Most infectious diseases are spread through indirect contact, such as pathogens carried through the air. These pathogens can be inhaled or picked up from surfaces.

Direct Contact Transmission

Some pathogens are spread by specific kinds of direct contact, such as:

• Sexual contact
• Drug use that involves shared syringes

Other Transmission Routes

Other infectious diseases are spread through:

• Contaminated water or food
• Animals to humans (known as zoonoses). The spread of zoonoses often involves vectors, which are disease carriers that usually do not get sick from the pathogen.

Nonspecific Defenses

The body has many nonspecific defenses, which defend against a wide range of pathogens.

First Line of Defense: Skin

Skin keeps pathogens out of the body by forming a barrier that few pathogens can penetrate. Mucus, saliva, and tears contain an enzyme that can kill bacteria. Mucus can also trap pathogens.

Second Line of Defense

When pathogens do enter the body, the second line of defense goes to work. These nonspecific defenses include:

• The inflammatory response, in which chemicals called histamines cause blood vessels near a wound to expand and phagocytes to move into the tissue to fight infection.
• The production of proteins called interferons, which help block the replication of viruses.
• The release of chemicals that produce a fever, an increase in normal body temperature, which may slow the growth of pathogens and speed up immune response.

Specific Defenses: The Immune System

The function of the immune system is to fight infection by inactivating foreign substances or cells that have entered the body. The specific immune response works in several ways, including:

• Recognizing "self," including cells and proteins that belong to the body.
• Recognizing "nonself," or antigens, molecules found on foreign substances. Antigens stimulate the immune system to produce cells called lymphocytes that recognize, attack, destroy, and "remember" specific pathogens.
• Producing specific lymphocytes that recognize specific antigens. They work by attacking infected cells or producing antibodies, proteins which tag antigens for destruction by immune cells.

The Immune System in Action

The immune response works in two ways:

• In humoral immunity, white blood cells, called B lymphocytes (B cells), make antibodies that attack pathogens in the blood.
• In cell-mediated immunity, white blood cells, called T lymphocytes (T cells), find and destroy abnormal or infected cells.

After a pathogen is destroyed, memory B cells and memory T cells stay in the body. These cells help create a faster immune response if the same pathogen enters the body again.

Acquired Immunity

You can acquire immunity without having a disease. Vaccination is the injection of a weakened or mild form of a pathogen to cause immunity.

• Active immunity results from vaccines or natural exposure to an antigen.
• Passive immunity forms when antibodies are introduced into the body. It lasts only until the immune system destroys the foreign antibodies.

Autoimmune Diseases

When the immune system makes a mistake and attacks the body's own cells, an autoimmune disease results. Autoimmune diseases include Type I diabetes, rheumatoid arthritis, and lupus.