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]

What the historical graph shows: The Vienna maternity-ward time series has persistently higher maternal mortality in the doctors'/medical-students' clinic than in the midwives' clinic, followed by an abrupt fall after chlorinated-lime hand cleansing was introduced in 1847. The graph is an early visual demonstration that interrupting transmission can change outcomes even before a pathogen is fully characterised. ^[1]

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]

Historical lecture distribution: The following proportions and the statement that HAI affected about 8% of inpatients are slide-era teaching figures, not current universal prevalence estimates. HAI burden varies by population, setting, case definition and surveillance method. ^[1][3][10]

HAI organisms may be antimicrobial-resistant: MRSA, ESBL-producing Enterobacterales, carbapenemase-producing Enterobacterales (CPE), carbapenem-resistant Acinetobacter baumannii, carbapenem-resistant Pseudomonas aeruginosa, and Candida auris. Clostridioides difficile is an important healthcare-associated diarrhoeal pathogen but is not grouped with these organisms simply because it is “MDR.” ^[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) and then a susceptible portal of entry. Demonstrating that an organism remains recoverable from a surface does not, by itself, demonstrate efficient clinical transmission from that surface. ^[1][19]

How to read the surface-survival slide: In the controlled 20 °C experiment shown, recoverable SARS-CoV-2 persisted for up to 28 days on several non-porous materials and paper banknotes, but fell faster on cotton and at higher temperatures. This is a laboratory persistence result under fixed conditions—not proof that ordinary fomites are a major transmission route or that a “durable coating” prevents infection. In practice, use risk-based cleaning of high-touch surfaces with an appropriate product, dilution and labelled contact time. ^[1][19]

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

The lecture presents a traditional droplet-versus-aerosol model: larger particles tend to settle more quickly and may deposit directly on nearby eyes, nose or mouth, whereas smaller particles remain suspended longer and may be inhaled. Distance, ventilation, humidity, respiratory activity and particle size all influence risk. ^[1][11]

• 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]

Smaller respiratory particles can remain suspended and be inhaled, including beyond close range when ventilation is poor. However, infectious respiratory particles exist on a continuous spectrum: the WHO-led 2024 terminology does not endorse one particle-size cut-off separating “droplet” from “airborne” physics. It describes through-air transmission as inhalation or direct deposition. ^[11]

Reading the deposition diagram: The slide maps progressively smaller particles to deeper respiratory deposition (nasopharyngeal → tracheobronchial → pulmonary/alveolar). Treat its exact 20/6/5 µm boundaries as a teaching simplification; deposition also depends on aerodynamic diameter, airway anatomy and breathing pattern. ^[1][11]

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 lymphoblastic leukaemia (ALL; blasts 78%), admitted to C6 ward at QMH. After chemotherapy and neutropenia, he developed sudden right-lower-quadrant pain, fever, confusion and hypotension; laparotomy showed necrotic bowel, and Rhizopus microsporus was identified. ^[1]

Visual recognition from the case slides: The resected bowel is dusky/necrotic. Histology shows broad, irregular, ribbon-like, pauciseptate hyphae—an appearance that should prompt concern for invasive Mucorales in the correct host. The culture becomes rapidly cottony/fluffy, and microscopy shows a sporangium with sporangiospores and rhizoids consistent with Rhizopus. Tissue invasion and vascular thrombosis explain the bowel necrosis; morphology is presumptive and culture or molecular identification establishes the organism. ^[1]

Attack rate on the slide = (mucosal + invasive)/total = 6/18 = 33.3%. Only 1 of 8 symptomatic patients survived (12.5% survival). The later bar chart is a historical projection, not an observed one-year outcome: extrapolating the detected rate gives 54 total, 36 asymptomatic and 18 symptomatic cases, from which the slide estimates about 16 lives saved by intervention. ^[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]

WHO “Five Moments” shown in the handout: before touching a patient; before a clean/aseptic procedure; after body-fluid exposure risk; after touching a patient; and after touching patient surroundings. ^[3]

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

The slide's “droplet > 5 µm / airborne < 5 µm” table is a historical simplification, and its “gloves for all patient contact” wording is broader than risk-based standard precautions. Precaution labels remain useful operational bundles, but do not imply a hard physical size boundary. ^[1][11][12]

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]

N95 fit testing and seal check: Fit testing is required before use and periodically under the applicable occupational-health programme (at least annually under US OSHA rules), and again when the respirator model/size changes or facial changes may affect fit. The handout uses ±10% weight change as a local trigger. A user seal check is still required every time the respirator is donned; it does not replace formal fit testing. ^[3][12]

Proper N95 Application Steps: ^[1]

1. Select regular or small-size mask according to face shape; loosen neck strap; ensure metal flexible strip faces upward
2. Place straps separately over the top of the head and behind the neck
3. Using index and middle fingers of both hands, press the metal strip simultaneously from centre toward both sides
4. Perform a fit check every time

Historical context only: The slide's US healthcare-worker mortality estimates are from 2000–2002 and should not be presented as current risk. They estimate 80–260 infection-related versus 77–93 injury-related deaths among 6.2 million healthcare workers, with HBV accounting for most of the estimated infection-related deaths. ^[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 → reconsider specimen processing and organisms not recovered by a routine short-incubation bacterial workflow
• Within 2 weeks, 35 similar cases at the same hospital
• The tissue image shows dense neutrophilic inflammation; the special stain shows sparse, slender, beaded acid-fast bacilli, pointing toward a rapidly growing nontuberculous mycobacterium rather than ordinary pyogenic bacteria ^[1]
• The published Shenzhen Women and Children's Hospital outbreak organism was Mycobacterium chelonae, not M. abscessus. “Rapidly growing” mycobacteria can produce visible growth within about 7 days under suitable culture conditions; routine workflows may be discarded too early or lack the appropriate work-up ^[18][20]
• Speciation requires mycobacterial culture and/or molecular methods. Do not infer “instruments or water” from this image alone; outbreak investigation must link the organism to a source/process. ^[18]

• F/37, assistant nurse in a CU clinic, presented with malaise, fever × 1 month, weight loss, and night sweats ^[1]
• CXR: Left-sided massive pleural effusion with collapse
• Sputum AFB: Positive → diagnosed with open pulmonary TB
• Known HBsAg carrier; baseline LFTs normal; albumin 20 g/L; started full-dose anti-TB treatment (body weight 45 kg)
• Over subsequent years, serial tables show resistance accumulating to isoniazid, rifampin, ethambutol, pyrazinamide, streptomycin, kanamycin, ofloxacin, ethionamide, amikacin and capreomycin; the final slide reports susceptibility only to cycloserine ^[1]
• Outcome: Admitted aged 45 with end-stage respiratory failure and destroyed left lung; no effective chemotherapy available; died 3 days after admission ^[1]
• Teaching point: The radiographs visually progress from major left pleural/lung disease to a destroyed left lung. This historical case illustrates cumulative resistance and the need for rapid diagnosis, effective multidrug therapy, adherence support, airborne precautions and stewardship. The slide's phrase “extremely multidrug-resistant” is descriptive historical wording; do not map it automatically to the current WHO formal XDR-TB definition.

• 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]