The recognition in 1840s by Semmelweis that microbes can be transmitted within hospitals to susceptible patients and healthcare workers was the start of infection control. ^[3]

High Yield: Hand hygiene is now the most important measure in infection control. Semmelweis observed that puerperal fever (Group A Streptococcus) rates dropped dramatically when clinicians washed hands with chlorinated lime solution before going from the post-mortem room to the delivery room. ^[1][3]

Before SARS in 2003, infection control was often neglected — the lecture slide noting healthcare worker deaths (Dr Tse Yuen Man, Dr Cheung Sik Hin, etc.) emphasises that "nobody cares till 2003". The SARS experience transformed infection control from a neglected discipline into a central hospital function. ^[1]

High Yield — directly from the lecture slide: ^[1]

HAI organisms are often MDR: MRSA, ESBL-producing Enterobacteriaceae, carbapenemase-producing Enterobacteriaceae (CPE), carbapenem-resistant Acinetobacter baumannii, carbapenem-resistant Pseudomonas aeruginosa, Candida auris, Clostridium difficile. ^[3]

Direct contact: Person-to-person, skin-to-skin touch. The most common route of nosocomial transmission. Your fingers touch a colonised patient → you touch another patient or yourself → transmission.

Indirect contact: Touching a fomite (contaminated surface/object) recently contaminated by an infected person. SARS-CoV-2 can survive on surfaces (stainless steel, glass) for days — hence the rationale for surface disinfection and durable surface disinfection coatings. ^[1]

Clinical pathways of contact transmission: ^[1]

• Finger skin → skin: skin colonisation/infection
• Finger skin → mucosa (eye, nose, mouth): colonisation or acute respiratory infection
• Finger → GI tract (ingestion): GI colonisation or infectious diarrhoea

Droplets are particles > 100 µm (some sources say ≥ 5–6 µm); they are "sprayed" onto the face and fall to the ground within ~1–2 metres. ^[1]

• Sneezing/coughing can produce up to 40,000 droplets of 0.5–12 µm at velocities of 100 m/s ^[1]
• Speaking also generates large numbers of particles < 5 µm; emission rate correlates with loudness; some individuals are "speech super-emitters" ^[1]
• The nearer you are to the patient, the higher the density of fine aerosol ^[1]

Aerosols are particles < 5 µm; they remain suspended ("floating") in the air for hours and can be inhaled over long distances. ^[1]

High Yield — The "airborne trio" + SARS-CoV-2: ^[1]

Parenteral routes: arthropod bites, needlestick injuries, surgery, transfusion, tissue/organ transplant. ^[1]

Bloodborne viruses of greatest concern (directly tested, 2024 SAQ Q12f): ^[2]

• HBV
• HCV
• HIV

Case illustration — blood product contamination: The lecture shows a case where a remnant blood bag (red cell unit serial 9019776) was found contaminated with Pseudomonas fluorescens at 5×10⁸ CFU/mL, with Gram smear and culture positive. This illustrates that bacterial contamination of blood products (though uncommon with modern screening) can cause fulminant sepsis. ^[1]

• Index patient: 6-year-old boy, good past health, newly diagnosed acute lymphocytic leukaemia (ALL, blasts 78%), admitted to C6 ward at QMH. Received chemotherapy → became neutropenic → developed sudden RLQ pain, fever, confusion, hypotension → ICU admission → laparotomy revealed necrotic bowel with "cotton candy-like colonies" → Rhizopus microsporus identified. ^[1]

High Yield: All allopurinol samples cultured were positive for Rhizopus microsporus group (4 batches), with fungal count > 1000/gm. The allopurinol contained corn starch as an inactive ingredient — a natural substrate for Mucorales. ^[1]

Attack rate = (mucosal + invasive)/total = 6/18 = 33.3%. Only 1 of 8 symptomatic patients survived (12.5% survival). Projected over 1 year without intervention: 54 total cases, 36 asymptomatic, 18 symptomatic → ~16 lives saved by the outbreak investigation. ^[1]

A high initial peak suggests a single, shared "common source" (e.g. contaminated food/drug batch). Controlling the source stops the outbreak. Person-to-person outbreaks give a longer, flatter curve with propagating peaks. ^[3]

High Yield: Standard precautions are applicable to ALL patients, regardless of diagnosis. They reduce the risk of transmission even when the presence of an infectious agent is unsuspected. ^[1][3]

Apply whenever contact with blood, body fluids, non-intact skin, mucous membranes, and secretions (except sweat) is possible. ^[1]

High Yield: "Hand hygiene: single most important infection control measure against nosocomial transmission of microbes." ^[1]

When to perform hand hygiene: ^[1]

• After ALL patient encounters
• After touching potentially contaminated materials in patient environment
• When going from a contaminated part of a patient (e.g. wound) to a clean site (e.g. IV line) on the same patient

Applied when a specific diagnosis is suspected or documented, IN ADDITION to standard precautions. ^[1]

High Yield — Mask Layers: The surgical mask has 3 layers: (1) water-repellent outer layer, (2) microbe-filtering melt-blown polypropylene middle layer, (3) water-absorbent inner layer. Always perform hand hygiene before putting on a respirator and perform a fit check every time for N95. ^[1]

Occupational death rate data (US HCW, 2000–2002): Infection-related deaths (80–260 per 6.2 million HCW) exceeded injury-related deaths (77–93). The majority were from HBV (75–250 deaths); HCV, HIV, and TB contributed 5–10 deaths total. ^[1]

High Yield: ^[1]

• Led by an Infection Control Officer (clinical microbiologist or trained physician) ^[3]
• Infection Control Nurse (ratio: 1 per 250 beds in HK) ^[3]
• Roles include: annual IC plan, surprise ward visits, surveillance, outbreak investigation, education, needle-stick monitoring, antibiotic stewardship collaboration, environmental cleaning oversight ^[3]

The final outcome is determined by the interaction between the microbe and host defence/underlying disease. Infection control procedures: ^[1]

1. Reduce risk and delay onset of HAI
2. Prevent outbreaks or cross-infection (protect patients, staff, visitors)
3. Cost saving

• F/23, D20 post-C-section, persistent wound pain and discharge
• P/E: 37.5°C, inflamed wound, serous discharge, indurated wound edge
• Pus C/ST: WBC +++, No growth → think of organisms not growing on routine media
• Within 2 weeks, 35 similar cases at the same hospital
• Diagnosis: Mycobacterium abscessus (rapidly growing mycobacterium; requires special media and prolonged incubation; won't grow on routine bacterial culture within 48 hrs)
• This is an outbreak of surgical site infections — likely contaminated surgical instruments or water supply ^[1]

• Patient transfused a red cell unit → developed fulminant sepsis
• Remnant blood bag cultured Pseudomonas fluorescens at 5×10⁸ CFU/mL ^[1]
• Platelet concentrates are more commonly contaminated than red cells because they are stored at room temperature (cf. red cells at 2–6°C) ^[5]

Q1: Name the MOST COMMON MDR gram-positive bacterium transmitted by contact in hospital-acquired infections. A: MRSA (Methicillin-resistant Staphylococcus aureus) ^[1][2]

Q2: Name the MOST COMMON MDR gram-negative bacterium transmitted by contact in hospital-acquired infections. A: ESBL-producing or carbapenemase-producing Enterobacteriaceae (e.g. E. coli, Klebsiella pneumoniae) — also accept carbapenem-resistant Acinetobacter baumannii ^[1][2]

Q3: Name the MOST COMMON MDR yeast transmitted by contact in hospital-acquired infections. A: Candida auris ^[1][2]

Q4: Name the MOST COMMON bacterial cause of infectious diarrhoea transmitted by contact in HAI. A: Clostridioides (Clostridium) difficile ^[1][2]

Q5: Name three microbes commonly transmitted by airborne route in hospital. A: Mycobacterium tuberculosis, Varicella zoster virus, Measles virus (also accept SARS-CoV-2) ^[1][2]

Q6: Name three viruses well-known to be transmitted by blood products. A: HBV, HCV, HIV ^[1][2]

Q7: Thirty students develop vomiting without fever ~1 hour after eating egg-and-ham sandwiches from a street vendor. The most likely causative agent is: A. Norovirus B. Salmonella Enteritidis C. Staphylococcus aureus D. Clostridium perfringens E. Bacillus cereus A: C. Staphylococcus aureus — preformed enterotoxin in prepared food, short incubation (1–6h), vomiting-predominant, no fever ^[8]

Q8: The single most important infection control measure against nosocomial transmission is: A. Wearing gloves B. Wearing N95 respirator C. Hand hygiene D. Patient isolation E. Antibiotic stewardship A: C. Hand hygiene ^[1]

Q9: Which type of public health surveillance involves health authorities actively seeking out cases through universal community testing? A. Passive B. Active C. Sentinel D. Syndromic A: B. Active surveillance ^[4]

Q10: A returning traveller from Southeast Asia presents with pneumonia, high fever, and lymphopenia. Name two conditions that should prompt admission to an isolation ward. A: (1) Pulmonary tuberculosis, (2) Novel influenza / avian influenza (or MERS-CoV) — both require airborne precautions ^[6]