DCM 3101 • LWA 1 • Sub-topic 1.2
Factors influencing disease transmission and control
Transmission is the movement of an infectious agent from a reservoir or source to a susceptible host. Control means reducing incidence, prevalence, complications and transmission; elimination and eradication are more ambitious endpoints. A clinician controls disease at the bedside and through the health system by identifying the active link in the transmission cycle.
- Explain the epidemiological triad of agent, host and environment.
- Identify predisposing factors that increase acquisition or severe disease.
- Distinguish contact, droplet, airborne, food-borne, water-borne, blood-borne, sexual, vertical and vector transmission.
- Apply the chain of infection to a patient, household and health-care outbreak.
- Select primary, secondary and tertiary control measures and evaluate their limitations.
1. The epidemiological triad
Disease patterns emerge from interaction among the agent, the host and the environment. A wet climate may increase mosquito habitat; human drainage may reduce stagnant water; altitude and temperature may alter agent survival or vector development. When the interaction remains relatively stable, disease may be endemic. When one component changes, transmission can increase or collapse.
| Component | Factors | Clinical and public-health question |
|---|---|---|
| Agent | Infective dose, survival, virulence, invasiveness, toxin production, antigenic change and antimicrobial resistance. | How much exposure is required, how long does it survive, and can treatment reduce shedding? |
| Host | Age, sex, pregnancy, immunity, vaccination, nutrition, genetics, comorbidities, behaviour and access to care. | Who is susceptible, who is infectious, and who needs prophylaxis or priority protection? |
| Environment | Climate, season, housing, crowding, sanitation, water, food, vectors, occupation, travel, conflict and health-care systems. | What conditions allow persistence and which environmental change would interrupt transmission? |
2. Predisposing factors
| Factor | How it increases transmission or disease | Examples |
|---|---|---|
| Poor environmental sanitation | Allows faecal contamination, vectors and unsafe waste disposal. | Polluted water, open defecation, blocked drains, poor air quality. |
| Poor personal hygiene | Increases hand, fomite, food and contact transmission. | Infrequent handwashing, unsafe food handling, shared personal items. |
| Crowding and displacement | Increases close contact, reduces isolation and disrupts health services. | Refugee settlements, prisons, dormitories, mass gatherings. |
| Substance use and risky behaviour | May damage immunity, increase exposure or reduce adherence to prevention. | Smoking, harmful alcohol use, unsafe injections, unprotected sex. |
| Low vaccination and limited health access | Leaves susceptible hosts and delays diagnosis, treatment and contact tracing. | Missed immunisation, transport barriers, medicine stock-outs. |
| Climate and geography | Changes vector density, water safety, agent survival and population movement. | Floods, drought, warm humid climates, forests and stagnant water. |
| Host vulnerability | Weakens barriers or immune responses. | Infancy, old age, pregnancy, malnutrition, diabetes, HIV and immunosuppression. |
| Socio-economic conditions | Shape housing, nutrition, occupation, sanitation and ability to seek care. | Poverty, overcrowded housing, hazardous work and food insecurity. |
3. Modes of transmission
| Mode | How it occurs | Control examples |
|---|---|---|
| Direct contact | Infectious blood, body substances, skin or mucosa contact another person or animal directly. | Gloves when indicated, wound covering, condoms, hand hygiene and safe sharps. |
| Indirect contact | Hands, surfaces, utensils, linen, phones, keyboards, taps or instruments pass organisms. | Cleaning, disinfection, sterilisation, equipment separation and hand hygiene. |
| Droplet | Larger respiratory particles from cough, sneeze, talk or splash reach nearby mucosa. | Source control, masks, distance, ventilation and respiratory etiquette. |
| Airborne | Small aerosols or droplet nuclei remain suspended and are inhaled over distance or time. | Ventilation, respirators, appropriate isolation and reduced crowding. |
| Food-borne | Consumption of contaminated or inadequately cooked food. | Safe preparation, temperature control, hand hygiene and inspection. |
| Water-borne | Consumption or use of contaminated water. | Safe water, chlorination, protected wells, sanitation and outbreak response. |
| Vector-borne | A mosquito, tick, tsetse fly or other vector carries the agent biologically or mechanically. | Bed nets, repellents, indoor spraying, drainage, protective clothing and case treatment. |
| Blood-borne and sexual | Blood, semen, vaginal fluid or other infectious material enters tissue or mucosa. | Condoms, testing, safe injections, screened blood, PrEP/PEP where indicated. |
| Vertical | Mother-to-child transmission during pregnancy, delivery or breastfeeding. | Antenatal testing, maternal treatment, safe delivery and infant prophylaxis. |
4. The disease-transmission cycle
The cycle links pathogen, reservoir, portal of exit, mode of transmission, portal of entry and susceptible host. A respiratory pathogen may leave through the nose, travel in droplets, enter another person’s airway and establish infection. A diarrhoeal pathogen may leave in stool, contaminate water or food, enter through the mouth and multiply in the intestine. The same principle guides both clinical precautions and community control.
- Agent: effective treatment, sterilisation and disinfection.
- Reservoir: diagnose, treat, isolate, clean the environment and manage animal sources.
- Portal of exit: masks, cough etiquette, wound covering, safe toilets and sharps safety.
- Transmission: hand hygiene, PPE, ventilation, food and water safety, vector control.
- Portal of entry: vaccination, barrier methods, aseptic technique and protective equipment.
- Susceptible host: vaccination, prophylaxis, nutrition, chronic-disease control and early care.
5. Airborne and droplet transmission in practice
Droplets can deposit on the eyes, nose or mouth after sneezing, coughing, talking or splashes. Airborne aerosols remain suspended longer and may be inhaled in poorly ventilated spaces. The distinction is not merely a label: it determines room ventilation, distance, mask or respirator selection, patient placement and duration of precautions. Influenza, mumps, rubella and meningococcal disease illustrate droplet risk; tuberculosis and measles require airborne thinking.
6. Contact transmission and fomites
Direct contact occurs when infectious material reaches another person through skin, mucosa, wounds or sexual contact. Indirect contact occurs through inanimate objects such as phones, keyboards, taps, linen, utensils or poorly processed instruments. Health-care workers can become a bridge between patients; hand hygiene at the correct moments and cleaning of shared equipment are high-value interventions.
7. Control and prevention
| Level | Purpose | Examples |
|---|---|---|
| Primary prevention | Prevent exposure or initial infection. | Vaccination, safe water, sanitation, condoms, vector control, ventilation, nutrition and hand hygiene. |
| Secondary prevention | Detect early disease and interrupt further spread. | Screening, testing, contact tracing, isolation, notification, prophylaxis and prompt treatment. |
| Tertiary prevention | Reduce complications, disability, relapse and onward transmission from chronic disease. | Rehabilitation, adherence support, chronic infection care and prevention of organ damage. |
| Control programme | Reduce incidence, prevalence or impact to a planned level. | Surveillance, case management, vaccination campaigns and quality improvement. |
| Elimination | Reduce transmission to zero in a defined geographic area. | Requires continued surveillance and rapid response to imported cases. |
| Eradication | End global transmission so routine control measures are no longer needed. | An absolute global endpoint, not a synonym for local control. |
8. Health-care infection prevention
- Apply standard precautions to every patient: hand hygiene, risk-assessed PPE, respiratory hygiene, sharps safety and safe handling of linen and waste.
- Use transmission-based precautions when contact, droplet or airborne spread is suspected.
- Clean and disinfect environmental surfaces; sterilise critical instruments that enter sterile tissue or the vascular system.
- Use aseptic technique for injections, wound care, catheter insertion and specimen collection.
- Reduce device days, review catheters daily and remove unnecessary invasive devices.
- Protect staff through vaccination, training, post-exposure protocols and a culture of reporting.
9. Outbreak control
- Verify the diagnosis and confirm that cases exceed the expected baseline.
- Develop a practical case definition and line list by person, place and time.
- Notify the public-health team, collect appropriate specimens and identify the likely source.
- Describe the outbreak with an epidemic curve, map and exposure timeline.
- Isolate or cohort cases, protect staff, trace contacts and give prophylaxis or vaccination when indicated.
- Control food, water, vectors, equipment or other environmental sources.
- Communicate transparently, correct misinformation and continue surveillance until transmission stops.
10. Applying the triad to a Ugandan clinical example
After heavy rains, a community reports febrile illness. The agent may be a malaria parasite or another pathogen; the host factors include children, pregnancy, malnutrition and lack of immunity; the environment includes stagnant water, mosquito density, housing and access to nets and testing. Control therefore needs clinical diagnosis and treatment, bed nets, drainage, indoor residual spraying, community education and surveillance—not a single intervention.
- Calling all respiratory spread airborne without considering aerosol-generating procedures and ventilation.
- Focusing on the patient while ignoring the reservoir, household, equipment or water source.
- Giving health education without removing structural barriers such as lack of water, masks or medicines.
- Stopping surveillance immediately after case numbers fall.
References
- SlideShare: Disease transmission and control
- WHO infection prevention and control resources.
- Uganda Clinical Guidelines, Ministry of Health.
