Epidemiology of Communicable and Non-Communicable Diseases
Epidemiology studies the distribution and determinants of health events and applies this knowledge to prevention and control. Communicable diseases result from infectious agents or their products and can be transmitted from a reservoir to a susceptible host. Non-communicable diseases (NCDs) are generally not spread from person to person and usually develop through interacting genetic, physiological, environmental and behavioural factors.
Learning objectives
- Differentiate communicable diseases from NCDs.
- Describe their distribution, determinants and natural history.
- Compare surveillance, prevention and control approaches.
- Explain the epidemiological transition and double burden of disease.
- Apply person-place-time analysis to both disease groups.
Core comparison
| Feature | Communicable diseases | Non-communicable diseases |
|---|---|---|
| Cause | Pathogenic agents such as bacteria, viruses, fungi, parasites or prions | Multiple genetic, metabolic, behavioural, environmental and social factors |
| Transmission | Directly or indirectly from a reservoir/source to a susceptible host | Usually no person-to-person transmission, though risk factors may cluster socially |
| Time course | Often acute, but some infections are chronic or latent | Usually gradual and prolonged, but may present acutely |
| Control focus | Interrupt transmission, treat cases, protect contacts and reduce reservoirs | Reduce shared risk factors, screen appropriately and provide long-term care |
| Examples | Malaria, tuberculosis, HIV, measles, cholera | Cardiovascular disease, cancer, diabetes, chronic respiratory disease |
Epidemiology of communicable diseases
Agent-host-environment interaction
- Agent: infectivity, pathogenicity, virulence, dose, resistance and antigenic characteristics.
- Host: age, immunity, nutrition, genetics, behaviour, occupation and comorbidity.
- Environment: climate, sanitation, housing, crowding, vectors, health services and social conditions.
Chain of infection
- Infectious agent
- Reservoir
- Portal of exit
- Mode of transmission
- Portal of entry
- Susceptible host
Control measures are selected to break one or more links—for example, effective treatment reduces the reservoir, safe water interrupts faeco-oral transmission and vaccination reduces host susceptibility.
Important epidemiological concepts
Infectivity
Ability of an agent to enter, survive and multiply in a host.
Pathogenicity
Proportion of infected people who develop clinical disease.
Virulence
Severity of disease among clinical cases, often reflected by complications or case fatality.
Incubation period
Time between infection and appearance of symptoms.
Epidemiology of non-communicable diseases
NCDs commonly arise through long causal pathways. Risk exposure may begin in childhood, biological changes develop silently and clinical disease appears years later. Prevention therefore requires action across the life course.
Four major NCD groups
- Cardiovascular diseases
- Cancers
- Chronic respiratory diseases
- Diabetes mellitus
Major modifiable risk factors
Behavioural
- Tobacco use
- Harmful alcohol use
- Unhealthy diet
- Physical inactivity
Metabolic
- Raised blood pressure
- Overweight and obesity
- Hyperglycaemia
- Dyslipidaemia
Environmental
- Air pollution
- Occupational hazards
- Unsafe built environments
- Climate-related exposures
Non-modifiable
- Age
- Genetic susceptibility
- Family history
- Previous disease
Person, place and time
| Dimension | Communicable example | NCD example |
|---|---|---|
| Person | Malaria risk by pregnancy, age and bed-net use | Hypertension by age, diet, obesity and family history |
| Place | Cholera clustering near unsafe water sources | Respiratory disease near polluted workplaces or roads |
| Time | Seasonal malaria peaks after rainfall | Long-term rise in diabetes with urbanisation |
Epidemiological transition and double burden
As populations change, patterns often shift from predominantly infectious, maternal, neonatal and nutritional conditions toward NCDs and injuries. The transition is not complete or uniform. Many communities face a double burden: persistent communicable diseases together with rapidly rising NCDs. Some infections also cause NCD outcomes—for example, oncogenic infections may contribute to cancer.
Syndemic interaction
Diseases may interact with each other and with adverse social conditions. Diabetes can increase susceptibility to severe infection; chronic infection can worsen nutrition; poverty can increase exposure while limiting access to care. Integrated services are therefore often more effective than isolated programmes.
Surveillance approaches
- Communicable disease: immediate alerts, case-based reporting, laboratory confirmation, contact investigation, outbreak thresholds and antimicrobial-resistance monitoring.
- NCD: mortality data, disease registries, facility records, household surveys, behavioural-risk surveillance and monitoring of metabolic indicators.
- Both: standard definitions, quality data, disaggregation by person/place/time and timely feedback for action.
Prevention and control
| Level | Communicable disease | Non-communicable disease |
|---|---|---|
| Primordial | Safe urban planning, sanitation systems and strong vaccination policy | Healthy food policy, tobacco control and activity-supporting environments |
| Primary | Immunisation, hand hygiene, vector control and prophylaxis | Risk-factor reduction, healthy diet, exercise and exposure control |
| Secondary | Early case detection, testing and prompt effective treatment | Evidence-based screening and early management of hypertension or diabetes |
| Tertiary | Management of complications and rehabilitation | Long-term disease control, complication prevention and rehabilitation |
Integrated public-health response
- Measure burden and identify high-risk groups.
- Address shared social and environmental determinants.
- Strengthen primary health care and referral systems.
- Ensure essential medicines, diagnostics and continuity of care.
- Use community participation and culturally appropriate communication.
- Monitor coverage, outcomes and equity.
Exam-focused summary
- Communicable diseases involve an infectious agent, reservoir, transmission route and susceptible host.
- NCDs usually develop through multiple interacting risks over a long period.
- Person-place-time analysis applies to both groups.
- Many low- and middle-income settings face a double burden of communicable disease and NCDs.
- Effective control combines clinical services, surveillance and action on social determinants.
Review questions
- Differentiate communicable and non-communicable diseases.
- Describe the agent-host-environment model for an infectious disease.
- List four major NCD groups and their shared modifiable risks.
- Explain epidemiological transition and double burden.
- Compare surveillance and prevention strategies for both disease groups.
References and further reading
Educational use only. Apply current Uganda Ministry of Health clinical, surveillance and public-health guidelines.
