Infectious Diseases: Pathogens, Spread, and Immunity

Posted on Apr 4, 2025 in Biology

Epidemics and Pandemics

Epidemic: An infectious disease outbreak that spreads quickly, affecting many people within the same region.

Pandemic: An epidemic that affects multiple countries or continents.

Emerging and Re-emerging Diseases

Emerging Diseases: Diseases caused by previously unknown germs.

Re-emerging Diseases: Diseases that reappear after a significant decline. Examples include:

  • Polio
  • Tuberculosis
  • Dengue
  • Malaria

Germ Theory

The theory that each infectious disease is caused by a specific microorganism, and each type of microorganism produces a distinct disease.

Infectious Agents (Pathogens)

Parasites that invade a host organism, reproduce, and cause tissue damage. They are responsible for symptoms; a set of symptoms constitutes a syndrome.

Types of Infectious Agents:

  • Viruses: Acellular parasites, the smallest infectious agents. They require host cells to reproduce.
  • Bacteria: Single-celled prokaryotic organisms capable of reproducing independently without invading other cells.
  • Protozoa and Fungi: Protozoa are unicellular eukaryotic organisms. Fungi can be unicellular or multicellular eukaryotes.

Cell Types:

  • Eukaryotic Cells: Form the bodies of animals, plants, and fungi, and also constitute single-celled protozoa.
  • Prokaryotic Cells: The type of cell found only in bacteria.

Sources of Infection and Transmission

Sources of Infection: Living beings infected by a pathogen.

Reservoirs: Animals that harbor infectious agents transmissible to humans.

Modes of Transmission:

  • Direct Transmission: Occurs through direct contact between an infected and a healthy individual, either physically or via respiratory particles.
  • Indirect Transmission: Germs pass from the source of infection to the environment (e.g., surfaces, water, air) before reaching a healthy individual.

Immunity and Immune Memory

Immune Memory: The immune system’s ability to remember a specific germ (pathogen) and react more quickly and intensely upon subsequent exposure. Immunity can be natural or artificial.

Vaccination: A process where pathogens (weakened or inactivated) are introduced into the body. These pathogens have lost their ability to cause infection but retain the ability to stimulate an immune response.

Antigenic Response: The process where organisms or molecules (antigens) are recognized by the immune system, stimulating its response.

Antibodies: Proteins produced by specialized immune cells (lymphocytes) that target specific antigens.

Treatments: Antibiotics and Antivirals

Antibiotics: Chemical substances, either biological or synthetic, that kill bacteria or prevent their multiplication.

Antivirals: Medications that act against viruses. Note: Antibiotics are ineffective against viruses.

Drug Resistance

The phenomenon where bacteria acquire resistance to an antibiotic.

Mechanisms of Resistance:

  • Mutation: Spontaneous changes in a bacterium’s genetic information can confer the ability to survive antibiotic exposure.
  • Gene Exchange: Bacteria can transfer resistance genes to other bacteria, spreading resistance.

Factors Promoting Resistance:

  • Inappropriate Treatment: Overuse or incomplete courses of antibiotics facilitate the selection and spread of resistant bacterial strains.
  • Use of Antibiotics in Agriculture: Widespread use in livestock and plants contributes to the pool of resistant bacteria.

How Influenza Virus Multiplies

a) The virus’s genetic information is distributed across 8 small RNA molecules.

b) The viral envelope contains two types of anchored molecules that define the virus type.

  1. The virus attaches to a host cell membrane.
  2. The virus enters the cell via endocytosis.
  3. The vesicle membrane and the virus envelope fuse, releasing the viral RNA into the cell.
  4. The host cell’s machinery is used to make copies of the viral genetic information (RNA) and proteins.
  5. New viruses assemble and emerge from the cell, ready to infect new cells.

The Immune Response Process

  1. A germ (pathogen) is detected and engulfed (phagocytosed) by immune cells like macrophages.
  2. Parts of the germ (antigens) are displayed on the surface of the macrophage and presented to lymphocytes (T cells and B cells).
  3. The lymphocytes receive signals that stimulate them to multiply.
  4. B cells produce specific antibodies against the germ.
  5. Certain T cells (cytotoxic T cells) detect and destroy infected host cells.

Key Features of the Immune Response:

  • Specificity: Lymphocytes and antibodies are effective only against the specific antigen that triggered their production.
  • Diversity: The immune system can respond to a vast array of different molecules (antigens).
  • Self/Non-self Discrimination: The ability to distinguish between the body’s own components (“self”) and foreign invaders (“non-self”).
  • Memory: The system remembers encountered pathogens, allowing for a faster and stronger response upon re-exposure.