Infectious Agents

Introduction to Infectious Diseases

• Infectious diseases present a significant global health challenge.

• Infectious agents range from extremely small viruses to large helminths (worms).

• The video will categorize these agents by size and list example diseases associated with them.

Scale of Measurement

• Nanoscale: 1 nm to 500 nm

  • Requires electron microscopy for visualization.

• Micron Level: 1 micron = 1000 nm, up to 200 microns

  • Requires light microscopy for visualization.

• Millimeter to Meters: 1 mm = 1000 microns

  • Visible to the naked eye.

Classes of Infectious Agents

Prions

• Size: Less than 20 nm.

• Description: Abnormally folded proteins that affect normal proteins.

• Diseases: Mad cow disease, Creutzfeldt-Jakob disease.

Viruses

• Size: 200 to 400 nm.

• Description: Intracellular parasites consisting of nucleic acids (DNA or RNA) encapsulated in a protein coat.

• Diseases: HIV, polio, COVID-19, hepatitis.

Bacteria

• Size: Approximately 0.2 to 15 microns.

• Description: Prokaryotic cells with membrane-bound organelles.

• Diseases: Tuberculosis, chlamydia, pneumonia.

Protozoa

• Size: 1 to 50 microns.

• Description: Single-celled eukaryotes with a defined nucleus.

• Diseases: Malaria, amoebic dysentery.

Fungi

• Size: 2 to 200 microns.

• Description: Eukaryotes that can be unicellular or multicellular, possessing a cell wall.

• Diseases: Candida (leading to thrush).

Helminths

• Size: Approximately 3 mm to 10 m.

• Description: Multicellular worms categorized into roundworms, tapeworms, and flukes.

• Diseases: Diverse diseases associated with different types of helminths.

Introduction to Infectious Diseases

Infectious diseases present a significant global health challenge, affecting millions worldwide and leading to considerable morbidity and mortality each year. Infectious agents vary in size and complexity, ranging from extremely small viruses to large multicellular organisms called helminths (worms). The video will categorize these agents by size, highlighting the unique characteristics and diseases associated with each class.

Scale of Measurement

• Nanoscale: 1 nm to 500 nmRequires electron microscopy for visualization, as these dimensions are considerably smaller than what standard light microscopy can achieve.

• Micron Level: 1 micron = 1000 nm, up to 200 micronsThis scale allows for visualization with light microscopy, which is capable of detailing cellular structures and certain unicellular organisms.

• Millimeter to Meters: 1 mm = 1000 micronsOrganisms at this scale are visible to the naked eye, including larger multicellular parasites and some fungi.

Classes of Infectious Agents

1. Prions

   • Size: Less than 20 nm

   • Description: Abnormally folded proteins that induce misfolding in normal proteins, leading to neurodegenerative diseases. Prion diseases are notoriously resistant to conventional sterilization processes, increasing their public health threat.

   • Diseases: Mad cow disease (BSE), Creutzfeldt-Jakob disease (CJD), and others that affect the brain and lead to severe neurological decline.

2. Viruses

   • Size: 200 to 400 nm

   • Description: Intracellular parasites that consist of nucleic acids (DNA or RNA) encapsulated in a protective protein coat. Viruses can only replicate inside a host cell, hijacking the host's cellular machinery for their reproduction.

   • Diseases: Notable viral infections include HIV, polio, COVID-19, hepatitis, and influenza, each presenting unique challenges in terms of treatment and prevention strategies.

3. Bacteria

   • Size: Approximately 0.2 to 15 microns

   • Description: Prokaryotic cells characterized by the absence of membrane-bound organelles. They have diverse metabolic pathways, enabling them to inhabit various environments.

   • Diseases: Pathogenic bacteria can cause diseases such as tuberculosis, chlamydia, pneumonia, and strep throat, and they can exhibit antibiotic resistance, complicating treatment.

4. Protozoa

   • Size: 1 to 50 microns

   • Description: Single-celled eukaryotes with a defined nucleus and various complex life cycles. Many protozoa are motile and can thrive in diverse environments, from freshwater to soil.

   • Diseases: Common diseases include malaria (transmitted by mosquitoes), amoebic dysentery, and giardiasis, which can lead to significant intestinal distress.

5. Fungi

   • Size: 2 to 200 microns

   • Description: Eukaryotic organisms that can be unicellular (yeasts) or multicellular (molds). They play crucial roles in ecosystems, including decomposition, but can also be pathogenic.

   • Diseases: Examples include candidiasis (leading to thrush), athlete's foot, and fungal infections in immunocompromised individuals, highlighting their importance in public health.

6. Helminths

   • Size: Approximately 3 mm to 10 m

   • Description: Multicellular worms categorized into roundworms, tapeworms, and flukes. They have complex reproductive systems and can live for extended periods within host organisms.

   • Diseases: Helminthic infections can lead to a variety of diseases including schistosomiasis, ascariasis, and hookworm infections, predominantly affecting developing countries and contributing to malnutrition and developmental issues in children.