Overview
This lecture covers the essential concepts of infection, including types of microorganisms, transmission methods, host resistance factors, infection control, and diagnostic methods.
Types of Microorganisms
- Microorganisms are very small living forms; major types: bacteria, fungi, viruses, protozoa.
- Bacteria and viruses cause most human infections.
- Bacteria: prokaryotic, no nucleus, can survive without a host, have complex cell walls, some form spores.
- Spores are dormant bacterial forms resistant to heat/disinfectants; example: Clostridioides difficile (C. diff).
- Viruses: intracellular parasites, use DNA or RNA, replicate by hijacking host cell machinery, mutate often.
- Some viruses cause latent infections (herpes zoster—shingles).
- Fungi: eukaryotic, contain a nucleus, commonly cause skin/mucous membrane infections, usually opportunistic.
- Helminths: parasitic roundworms/flatworms, enter via ingestion or contaminated sources, examples: pinworms, hookworms, tapeworms.
Normal Flora & Infection Principles
- Normal (resident) flora are non-pathogenic organisms in body areas (gut, mouth, skin) that aid in digestion and waste processing.
- Infection occurs when microorganisms invade and reproduce in body tissues.
- Terms: sporadic (isolated cases), endemic (regular in a population), epidemic (above-normal cases), pandemic (global spread).
Transmission of Infection
- Reservoir: source of infection (person, animal, environment).
- Carrier: person who harbors and transmits disease without symptoms.
- Portal of exit: how pathogen leaves reservoir (cough, secretions).
- Mode of transmission: direct contact, indirect contact (fomites), droplet, aerosol, vector-borne.
- Portal of entry: how infection enters new host (airway, skin, GI tract).
- Susceptible host: individuals at higher risk due to age, immune status, or health conditions.
Nosocomial Infections & Host Resistance
- Nosocomial infections: acquired in healthcare facilities, common due to concentrated microbes and vulnerable patients.
- Decreased host resistance factors: very young/old age, pregnancy, immunodeficiency, chronic illness, poor nutrition, severe stress.
Infection Control & Prevention
- Standard precautions: applied uniformly to all patients to prevent transmission (gloves, hand hygiene).
- Specific precautions: used for diagnosed diseases (droplet, airborne precautions).
- Breaking transmission: isolate reservoirs, restrict contaminated sources, block portals, practice proper hygiene and cleaning.
Diagnosis & Physiology of Infection
- Incubation period: time from organism entry to symptom onset.
- Prodromal period: onset of general symptoms (malaise, fatigue).
- Acute period: disease fully develops with specific symptoms.
- Local infection signs: pain, swelling, redness, drainage.
- Systemic signs: fever, malaise, headache, elevated white count.
- Diagnosis: cultures, drug sensitivity, blood tests (leukocytosis for bacterial, leukopenia for viral), C-reactive protein, ESR, immunological tests.
Key Terms & Definitions
- Pathogen — disease-causing microorganism.
- Normal flora — beneficial microorganisms naturally present in the body.
- Reservoir — source where infectious agent lives and multiplies.
- Carrier — person who transmits disease without symptoms.
- Sporadic/Endemic/Epidemic/Pandemic — terms describing spread patterns of diseases.
- Fomite — inanimate object that can carry infectious agents.
- Nosocomial infection — infection acquired in a healthcare setting.
Action Items / Next Steps
- Review Chapter 6 notes and key terms.
- Study the differences between types of microorganisms and modes of transmission.
- Prepare for Chapter 7 on immunity.