Pathogens: Understanding Disease-Causing Microorganisms

A pathogen is any microorganism capable of causing disease. Here are the major classes.

1. Bacteria (Prokaryotes)

• Salmonella (foodborne gastroenteritis, typhoid)
• E. coli (UTI, diarrheagenic strains: EHEC, ETEC)
• Staphylococcus aureus (skin/soft tissue, toxic shock, MRSA)
• Streptococcus pneumoniae (pneumonia, meningitis, otitis)
• Mycobacterium tuberculosis (TB)
• Clostridioides difficile (antibiotic-associated colitis)

2. Viruses (Obligate Intracellular)

• Influenza (seasonal flu, pandemic potential)
• HIV (AIDS — CD4+ T cell depletion)
• Hepatitis B/C (chronic liver disease, HCC)
• Measles (highly contagious, vaccine-preventable)
• SARS-CoV-2 (COVID-19)
• HPV (cervical/oropharyngeal cancer)

3. Fungi (Eukaryotes)

• Candida albicans (mucocutaneous, invasive candidiasis)
• Aspergillus fumigatus (allergic, invasive aspergillosis)
• Cryptococcus neoformans (meningitis in immunocompromised)
• Dermatophytes (tinea/ringworm)

4. Parasites (Eukaryotes)

• Plasmodium spp. (malaria — mosquito vector)
• Hookworm (Necator americanus) — intestinal blood loss
• Tapeworm (Taenia) — foodborne
• Toxoplasma gondii (congenital, reactivation in AIDS)
• Giardia (waterborne diarrhea)

Pathogenic Mechanisms

• Adhesion/invasion
• Toxin production (exo/endotoxin)
• Immune evasion (antigenic variation, intracellular hiding)
• Host damage (direct cytolysis, immunopathology)

Pathogenicity depends on microbe virulence factors AND host susceptibility (immune status, genetics, microbiome). Not all microbes are pathogens; many are commensal or beneficial.