Infectious agents and disease - An overview

Infectious Agents

• Definition: Agents that can cause injury, disease, or death to the host (human body).

• Pathogenic: Refers to agents that cause disease.

  • "Patho" means disease, and "genic" means causing.

• Virulence: A measure of how severe or harmful a pathogen is.

  • More virulent strains may evade the immune system or be more transmissible.

Impact of Infectious Agents

• In developed countries, the eighth leading cause of death is from infectious agents, causing diseases like influenza and pneumonia.

• During the COVID-19 pandemic, it became the third leading cause of death in developed countries.

• In the developing world, the top three causes of death from infectious agents are:

  1. Lower respiratory tract infections.

  2. HIV/AIDS.

  3. Diarrhea-related diseases.

• Other significant causes of death include malaria and tuberculosis.

Categories of Infectious Agents

1. Prions

• Size: Less than 20 nanometers.

• Definition: Infectious proteins that cause disease by misfolding normal proteins in the nervous system.

  • Misfolded proteins lead to neurological symptoms.

• Spread through blood transfusions, organ transplants, cannibalism, etc.

• Diseases associated: Transmissible spongiform encephalopathies (TSE), e.g., Mad Cow Disease (BSE), Creutzfeldt-Jakob Disease (CJD).

2. Viruses

• Size: 20 to 400 nanometers.

• Definition: Obligate intracellular parasites that require a host cell to replicate.

• Structure: Composed of a capsid (protective protein coat) with either RNA or DNA genome.

• Classification based on:

  • Type of genetic material (RNA or DNA).

  • Presence of an envelope.

• Disease examples:

  • Respiratory tract: Common cold, influenza, COVID-19 (SARS-CoV-2).

  • Gastrointestinal tract: Rotavirus, mumps.

  • Systemic: Chickenpox (varicella), HIV.

  • Skin: Human papillomavirus (HPV).

  • CNS: Polio.

3. Bacteria

• Size: 0.2 to 15 microns (200 to 15,000 nanometers).

• Definition: Prokaryotic, unicellular organisms with DNA in a nucleoid.

• Structure: Cell membrane and cell wall (peptidoglycan).

• Classification:

  • Gram-positive (thick cell wall) vs. Gram-negative (thin wall with outer membrane).

  • Shape: Cocci (spherical), bacilli (rod-shaped), staphylococci (clusters), streptococci (chains).

• Oxygen requirements: Aerobic (needs oxygen), anaerobic (does not need oxygen), facultative (can switch).

• Disease examples:

  • Obligate intracellular: Chlamydia (causes scarring, infertility).

  • Facultative intracellular: Mycoplasma (causes pneumonia).

  • Extracellular: Strep throat (Streptococcus), skin infections (Staphylococcus aureus), tetanus (Clostridium tetani).

  • Toxins: Gram-positive bacteria produce exotoxins; gram-negative bacteria release endotoxins.

4. Fungi

• Size: 2 to 200 microns.

• Definition: Eukaryotic organisms with membrane-bound organelles and a cell wall made of chitin.

• Reproduction: Can reproduce sexually and asexually through spores.

• Disease types:

  • Superficial infections: Ringworm (tinea), athlete's foot (tinea pedis), oral and vaginal thrush.

  • Deep infections: Serious conditions in immunocompromised individuals.

5. Protozoa (Protists)

• Size: 1 to 50 microns.

• Definition: Eukaryotic, usually unicellular organisms.

• Disease examples:

  • Malaria: Cycle between mosquito and human, affecting red blood cells.

  • Giardia: Contaminated food/water causing gastrointestinal distress, inflammation, diarrhea.

6. Helminths

• Size: 3 mm to 10 m.

• Definition: Multicellular worms or ectoparasites classified into roundworms, flatworms, and flukes.

• Examples:

  • Roundworms: Hookworms (infect gastrointestinal tract).

  • Flatworms: Tapeworms (often long).

  • Ectoparasites: Mites, fleas causing skin irritation; ticks can transmit bacteria causing diseases like Lyme disease.

Infectious Agents

Definition

• Infectious agents are biological entities that can invoke harm, injury, disease, or even death in a host, particularly in the human body. These agents are responsible for various illnesses and outbreaks globally.

Pathogenic Nature

• The term "pathogenic" describes agents that are capable of causing disease. The prefix "patho" means disease, while "genic" implies causing. Understanding pathogenicity is crucial for recognizing and treating infectious diseases.

Virulence

• Virulence is defined as the degree of pathogenicity within a microorganism, indicating how severe or harmful a pathogen can be. More virulent strains are characterized by their ability to evade the host's immune system, leading to increased transmissibility and severity of disease.

Impact of Infectious Agents

• In developed countries, infectious agents rank as the eighth leading cause of death, responsible for diseases such as influenza and pneumonia.

• Notably, during the COVID-19 pandemic, infections surged to become the third leading cause of death in these regions, highlighting the vulnerability of populations to infectious threats.

• In developing countries, the three primary causes of death linked to infectious agents include lower respiratory tract infections, HIV/AIDS, and diarrhea-related diseases. Additionally, malaria and tuberculosis are significant contributors to mortality rates.

Categories of Infectious Agents

1. Prions

   • Size: Less than 20 nanometers.

   • Definition: Infectious proteins that induce disease by misfolding normal proteins within the nervous system. Misfolded proteins disrupt normal cellular functions, leading to neurological symptoms such as cognitive decline and motor dysfunction.

   • Transmission: Spread through blood transfusions, organ transplants, and practices like cannibalism.

   • Associated Diseases: Transmissible spongiform encephalopathies (TSE) including Mad Cow Disease (BSE) and Creutzfeldt-Jakob Disease (CJD).

2. Viruses

   • Size: 20 to 400 nanometers.

   • Definition: Obligate intracellular parasites that can replicate only within a living host cell. Their survival relies on accessing host cellular machinery.

   • Structure: Composed of a capsid (protective protein coat) encasing either an RNA or DNA genome.

   • Classification: Based on the type of genetic material present (RNA or DNA) and whether an envelope surrounds the virus.

   • Disease Examples:

     • Respiratory tract: Common cold, influenza, COVID-19 (SARS-CoV-2).

     • Gastrointestinal tract: Rotavirus, mumps.

     • Systemic infections: Chickenpox (varicella), HIV.

     • Skin infections: Human papillomavirus (HPV).

     • CNS infections: Polio.

3. Bacteria

   • Size: 0.2 to 15 microns (200 to 15,000 nanometers).

   • Definition: Prokaryotic, unicellular organisms characterized by a lack of membrane-bound organelles and containing their DNA in a nucleoid region.

   • Structure: Comprises a cell membrane and a cell wall made predominantly of peptidoglycan.

   • Classification:

     • Gram-positive: Possess a thick cell wall, appearing purple in Gram staining.

     • Gram-negative: Feature a thin wall accompanied by an outer membrane, appearing pink in Gram staining.

   • Shapes: Cocci (spherical), bacilli (rod-shaped), staphylococci (clusters), and streptococci (chains).

   • Oxygen Requirements:

     • Aerobic: Require oxygen for growth.

     • Anaerobic: Grow without oxygen.

     • Facultative: Can grow in both conditions.

   • Disease Examples:

     • Obligately intracellular pathogens: Chlamydia (causes reproductive issues and infertility).

     • Facultatively intracellular: Mycoplasma (associated with pneumonia).

     • Extracellular infections: Strep throat (Streptococcus), skin infections (Staphylococcus aureus), tetanus (Clostridium tetani).

     • Toxins: Gram-positive bacteria often produce exotoxins, while gram-negative bacteria are known to release endotoxins upon cell lysis.

4. Fungi

   • Size: 2 to 200 microns.

   • Definition: Eukaryotic organisms possessing membrane-bound organelles and cell walls composed of chitin, unlike bacterial cell walls made of peptidoglycan.

   • Reproduction: Can reproduce through both sexual and asexual means, often via spores that allow for widespread dispersal.

   • Disease Types:

     • Superficial infections: Such as ringworm (tinea), athlete's foot (tinea pedis), and candidiasis (oral and vaginal thrush).

     • Deep infections: Can lead to severe complications, particularly in immunocompromised individuals who lack adequate immune defenses.

5. Protozoa (Protists)

   • Size: 1 to 50 microns.

   • Definition: Eukaryotic, predominantly unicellular organisms that can exist independently or as parasites.

   • Disease Examples:

     • Malaria: Involves a complex lifecycle with transmission between mosquitoes and humans, significantly impacting red blood cells.

     • Giardia: Transmitted through contaminated food or water, causing gastrointestinal distress characterized by inflammation and diarrhea.

6. Helminths

   • Size: 3 mm to 10 m.

   • Definition: Multicellular organisms categorized as worms or ectoparasites, sub-divisible into roundworms, flatworms, and flukes, each with distinct life cycles and modes of transmission.

   • Examples:

     • Roundworms: Hookworms, which invade the gastrointestinal tract.

     • Flatworms: Tapeworms, which can grow to substantial lengths within the host intestine.

     • Ectoparasites: Mites and fleas that cause skin irritation, while ticks are known vectors that can transmit pathogens leading to diseases such as Lyme disease.

Understanding the diverse categories and impacts of infectious agents is crucial for public health management, disease prevention, and treatment initiatives.