3.2 1.a - Classifying, mapping and tracing disease

Disease classifications

A disease classification groups conditions by a chosen characteristic. Different classifications answer different questions, so one disease can belong to several groups. Confusing its cause, transmission and geographical scale produces misleading explanations.

An infectious disease is caused by a pathogen, such as a virus, bacterium or parasite. A non-infectious condition is not caused by infection with a pathogen; type 2 diabetes is an example. Communicable diseases can be transmitted from an infected source or reservoir to a susceptible host, directly or indirectly. Infections such as tetanus show why the terms are not perfect synonyms: environmental spores can cause infection without ordinary person-to-person spread. Noncommunicable diseases are not passed between people in this way, although infections can contribute to some conditions, such as particular cancers.

Contagious usually describes disease readily transmitted between people through contact or respiratory pathways. A communicable disease can be non-contagious in ordinary contact: malaria is usually transmitted through infected mosquitoes rather than by sitting beside an affected person. HIV also is not transmitted by casual social contact. The relevant transmission mechanism must be identified rather than assuming that proximity always creates the same risk. WHO describes malaria's vector-borne transmission.

A second classification concerns occurrence. Endemic means a disease is persistently present at an expected level in a population or area. It does not mean harmless, equally distributed or impossible to control. An epidemic is occurrence above the expected level for a defined place and period. A pandemic involves extensive spread across countries or continents, usually with sustained transmission. These terms describe patterns, not a fixed death rate. A severe local outbreak is not automatically a pandemic, and a pandemic can affect groups with very different risks.

An illustrative example separates these dimensions. A mosquito-borne infection may be endemic in one region, generate an epidemic when cases exceed its usual seasonal level, and still remain non-contagious through everyday contact. Classification helps determine whether to investigate vector habitats, water contamination, close-contact networks or other pathways.

Global patterns of communicable disease

A disease map needs a defined measure, population and date. A large number of cases may reflect a large population, while a high rate indicates a greater burden relative to the population. Surveillance also matters: places with more testing may record more disease even when underlying infection is lower.

Malaria is concentrated in tropical and subtropical settings where suitable mosquitoes, parasites and human exposure coincide. Sub-Saharan Africa carries the dominant global burden; important transmission also occurs in parts of South and South-East Asia, Oceania and tropical Latin America. Cool highlands, dry conditions, vector control and healthcare can interrupt this broad pattern. WHO's World Malaria Report 2025 estimates about 282 million cases globally in 2024. That is an estimated flow of episodes during a year, not the number of people infected on a single day. WHO malaria evidence, 2025 report basis.

HIV occurs worldwide, with eastern and southern Africa the most affected region. This pattern cannot be explained by tropical climate alone: transmission networks, historical spread, inequality, prevention and access to sustained treatment matter. UNAIDS' 2024 regional assessment reported substantial falls in new infections and AIDS-related deaths between 2010 and 2023, despite the continuing high burden. Prevalence can remain high when treatment allows people to live longer, even as new infections decline. UNAIDS eastern and southern Africa profile, July 2024.

Tuberculosis is concentrated in much of South and South-East Asia, parts of the Western Pacific and sub-Saharan Africa. High absolute burdens occur in populous countries including India and Indonesia, while rates vary between and within countries. Crowded, poorly ventilated housing, undernutrition, HIV and barriers to diagnosis or treatment help explain the geography. An infection acquired earlier can progress to active disease later, so present disease patterns do not map recent travel alone. WHO Global Tuberculosis Report 2025 assessment.

These patterns reveal overlapping physical and human causes. A map is the beginning of explanation, not proof that one associated factor causes every case. Comparisons should distinguish diagnosed cases from modelled burden, use consistent age groups and note missing or uncertain observations.

Worked resource critique — a disease map

Illustrative resource: a world choropleth is titled “Risk of tuberculosis in 2024” but shades countries using raw reported case totals. Its legend says “low”, “medium” and “high” without numerical boundaries. White represents both zero cases and unavailable observations. Identify three limitations of the presentation technique. Raw totals do not standardise for population, so they cannot directly represent the individual risk implied by the title. Unspecified class boundaries prevent the reader identifying what separates the three categories. Combining missing observations with zero cases makes absent information look like absence of disease. Each limitation concerns a different property of the resource; the answer does not assume that a correctly labelled map lacks a date or country boundaries.