Microorganisms, Pathogens & Immunity Study Notes

Viruses

  • Smallest known infectious agents; obligate intracellular parasites → must invade living tissue (host cell) to multiply.
  • Pathogenesis sequence:
    • Attachment to host cell ➔ hijacks cellular machinery ➔ destroys host cell ➔ spreads by moving to adjacent cells.
  • Latency vs. rapid onset:
    • Some viruses cause immediate signs/symptoms.
    • Others lie dormant for years, then reactivate (e.g., Varicella-Zoster → shingles).
  • Laboratory challenges:
    • Requires living tissue cultures for growth/identification.
    • High mutation & replication rates ⟹ continual antigenic change ⟹ difficult to culture & develop lasting treatments.
  • Influenza case study:
    • Seasonal “flu” virus mutates yearly → annual vaccine reformulation.
    • Vaccination does not fully prevent infection; it lowers severity/complications.
  • Therapeutic limitation:
    • Medicine can often only reduce symptom severity; body’s immune system must ultimately clear virus.
  • Notable medical advance:
    • Modern antivirals for HIV can render viral load almost undetectable, yet virus persists in latent reservoirs.

Common Disease-Causing Viruses & Transmission Modes

  • Influenza – airborne/droplet.
  • Measles – airborne/droplet.
  • Epstein-Barr virus (mononucleosis) – saliva (“kissing disease”).
  • Hepatitis B – blood-borne or sexual.
  • HIV/AIDS – blood-borne or sexual.
  • Rabies – vector-borne (animal bite).
  • Rhinovirus (common cold) – droplet, direct contact, or fomites; droplet travel range 3\text{–}6\ \text{ft}.
  • Varicella-Zoster (chickenpox ➔ shingles) – airborne, droplet, or contact with vesicular fluid.
  • Smallpox – direct contact or fomites.

Fungi/Fungus

  • Terminology:
    • “Fungi” = plural; “fungus” = singular (often refers to mold form).
  • Eukaryotic organisms; may exist as single-celled yeasts or multicellular molds.
  • Habitat: grow on/in animals & plants; prefer warm, moist areas (e.g., between toes).
  • Most are non-pathogenic to healthy hosts; threat rises in immunocompromised individuals.
  • Human benefit: source of many antibiotics.

Common Disease-Causing Fungi

  • Candidiasis (thrush/vaginal yeast infection) – overgrowth of normal flora; white plaques in mouth replicate vaginal presentation.
  • Dermatophytes (ringworm/athlete’s foot) – direct contact (skin, showers, locker-room floors).

Protozoa

  • Single-celled parasites; size ranges from microscopic to barely visible without magnification.
  • Rapid reproduction inside host; thrive in moist environments (standing water, ponds, lakes).

Common Protozoan Diseases

  • Malaria – vector transmission via Anopheles mosquito.
  • Trichinosis – food-borne (undercooked pork).
  • Trichomoniasis – sexually transmitted (skin-to-skin mucosal contact).

Multicellular Parasites (Helminths & Ectoparasites)

  • Cause infestations → may progress to infection.
  • Examples/Transmission:
    • Lice (head, body, pubic) – contact with infested hair or personal items.
    • Bed bugs – contact with infested bedding/furniture.
    • Scabies (mite) – prolonged skin contact.
    • Helminths:
    – Tapeworm – food-borne (undercooked meat/fish).
    – Roundworm – soil-borne (contaminated soil ingestion).

Immunity Terminology

  • Immunocompetent – normal immune function; typical resistance to infection.
  • Immunodeficient / Immunocompromised / Immunosuppressed – diminished immune capability ➔ higher susceptibility & risk of opportunistic infections (pathogens usually harmless to healthy hosts).
    • At-risk groups: chemotherapy patients, organ-transplant recipients, advanced/untreated HIV, long-term immunosuppressive drug users.

Opportunistic Infection Definition

  • Infection by ordinarily non-pathogenic microbes that exploit weakened immune defenses.

Types of Immunity

Active Immunity

  • Natural: acquired through exposure to pathogen; body mounts immune response & memory.
  • Artificial: induced via vaccination. Lab-produced antigens teach immune system to respond if exposed later (e.g., hepatitis, varicella vaccines).

Passive Immunity

  • Natural: maternal antibodies transferred to infant, primarily via colostrum in early breast milk.
  • Artificial: injection/infusion of lab-produced monoclonal antibodies; immediate but temporary protection.

Diagnostic Specimen Collection

  • Provider assessment ➔ obtain specimens ➔ laboratory microscopy/culture to identify microorganism.
  • Specimen types:
    • Blood.
    • Urine.
    • Sputum.
    • Wound swab/culture (illustrated example: foot ulcer swab).
    • Throat swab.
  • Lab media (e.g., Petri dishes) allow growth/visualisation for definitive diagnosis & targeted therapy.