What Is a Pathogen? Definition, Types, and Transmission

True. A pathogen is any microorganism that can cause disease in a host.

Major Pathogen Types

• Bacteria: Single-celled prokaryotes (e.g., Streptococcus, Salmonella, Mycobacterium tuberculosis).
• Viruses: Obligate intracellular genetic parasites (e.g., influenza, HIV, SARS-CoV-2, norovirus).
• Fungi: Yeasts and molds (e.g., Candida, Aspergillus, dermatophytes).
• Protozoa: Single-celled eukaryotes (e.g., Plasmodium malaria, Giardia, Toxoplasma).
• Helminths (Parasitic Worms): Multicellular (e.g., roundworms, tapeworms, flukes).
• Prions: Misfolded infectious proteins (e.g., Creutzfeldt-Jakob disease).

Transmission Routes

• Direct Contact: Skin-to-skin, sexual, mother-to-child.
• Respiratory/Droplet/Airborne: Cough/sneeze (flu, TB, COVID-19).
• Fecal-Oral: Contaminated water/food/hands (cholera, hepatitis A, norovirus).
• Vector-Borne: Mosquitoes (malaria, dengue), ticks (Lyme), fleas (plague).
• Vehicle-Borne: Food, water, blood products, fomites.

Key Concept: Pathogenicity vs. Virulence

Pathogenicity = ability to cause disease. Virulence = degree of disease severity. Not all exposures lead to illness; host immunity, inoculum size, and pathogen factors determine outcome.