GCRI · Health Nexus

Infectious threats

An outbreak becomes an emergency when a pathogen spreads faster than the systems that detect, characterise and contain it. Readiness depends on surveillance that sees early, laboratories that confirm, and evidence that is honest about what is not yet known. This page sets out how infectious threats emerge and spread, how they are detected and assessed, and how uncertainty is carried into the decisions of public health authorities.

01Register

Infectious threats on the record.

02The question

Infectious threats: how the risk develops.

Outbreak readiness, surveillance and uncertainty. The dossier below sets out how the risk develops, what is measured, where the data come from, how it is modelled and who decides.

Mechanism

Detection lags transmission

Cases appear in surveillance only after infection, symptoms, care-seeking, testing and reporting. Each step adds delay and loses cases, so early counts describe the past and undercount the present.

Mechanism

Contact and immunity shape spread

Spread depends on the pathogen’s transmissibility, the timing of infectiousness, population immunity and contact patterns. Mobility, crowding and mass gatherings connect local chains of transmission into regional and international ones.

Mechanism

Pathogens evolve under pressure

Variants with altered transmissibility, severity or immune escape arise and are selected. Genomic surveillance detects them, but only where sequencing capacity exists and samples and data are shared promptly.

Mechanism

Outbreaks cascade into other systems

Surges strain hospitals and staff, travel and trade restrictions disrupt supply chains, and absenteeism reaches food, energy and transport. Public trust and the quality of information shape behaviour, and so the course of the outbreak itself.

03Geography

Environmental exposure in Africa.

Population density, daytime surface heat and haze over Africa, from NASA GIBS. Each layer carries its own date.

The interactive map needs JavaScript; the layer list and sources are below.

Layers 3

Environmental exposure, for this page

About this layer
Evidence
Estimated
Source
Gridded Population of the World v4.11 (NASA SEDAC (CIESIN, Columbia University))
Provider
NASA SEDAC (CIESIN, Columbia University)
Licence
CC BY 4.0
Vintage
null; Census-based estimate, static (2020)
Resolution
about 1 km grid
Uncertainty
Census-based estimate of people per square kilometre in 2020, shown as exposure context; not a count of people affected by any hazard.
Reached through
gibs.earthdata.nasa.gov, only after you allow the live map
About this layer
Evidence
Observed (persistent cloud still leaves gaps)
Source
VIIRS NOAA-20 Level 3, 8-day (NASA LANCE)
Provider
NASA LANCE
Licence
NASA open data
Vintage
8-day; Satellite 8-day composite; persistent cloud still leaves gaps
Newest at build
2026-09-14
Resolution
about 1 km grid
Uncertainty
Satellite skin temperature of the ground over the 8 days ending on the date shown, a heat exposure indicator; not air temperature, heat index or a heat warning.
Reached through
gibs.earthdata.nasa.gov, only after you allow the live map
About this layer
Evidence
Observed (clouds leave gaps)
Source
VIIRS NOAA-20 Deep Blue AOD (NASA LANCE)
Provider
NASA LANCE
Licence
NASA open data
Vintage
daily; Satellite retrieval; clouds leave gaps
Newest at build
2026-10-14
Resolution
about 2 km grid
Uncertainty
Satellite measure of haze, smoke and dust through the whole air column on the day shown; an exposure indicator, not ground-level PM2.5, an air quality index or a health warning.
Reached through
gibs.earthdata.nasa.gov, only after you allow the live map
About this layer
Evidence
Observed
Source
VIIRS NOAA-20 corrected reflectance (NASA LANCE)
Provider
NASA LANCE
Licence
NASA open data
Vintage
daily; Satellite image
Newest at build
2026-10-14
Resolution
about 250 m grid
Uncertainty
Satellite image. Satellite or model raster as the provider publishes it. Clouds, swath gaps and retrieval error leave gaps and noise; values are indicators, not measurements at a place.
Reached through
gibs.earthdata.nasa.gov, only after you allow the live map
About this layer
Evidence
Observed (clouds leave gaps)
Source
OMI Aura (NASA GES DISC)
Provider
NASA GES DISC
Licence
NASA open data
Vintage
daily; Satellite retrieval; clouds leave gaps
Newest at build
2026-10-10
Resolution
about 2 km grid
Uncertainty
Satellite column of nitrogen dioxide in the lower atmosphere on the day shown, mostly from combustion; not a ground-level concentration or a health warning.
Reached through
gibs.earthdata.nasa.gov, only after you allow the live map
About this layer
Evidence
Modelled (latest year GIBS publishes)
Source
MODIS Terra and Aqua land cover (MCD12Q1) (NASA LP DAAC)
Provider
NASA LP DAAC
Licence
NASA open data
Vintage
yearly; Satellite classification; latest year GIBS publishes
Newest at build
2024-01-01
Resolution
about 500 m grid
Uncertainty
Satellite land cover classes for the year shown, including croplands; a classification with known errors, not a farm register.
Reached through
gibs.earthdata.nasa.gov, only after you allow the live map

Latest available

NASA GIBS / EOSDIS · ASTER GDEM (METI, NASA) · Blue Marble (NASA) · © OpenStreetMap contributors (ODbL) · Natural Earth

Loading the latest NASA imagery.

Drag to move. Ctrl or ⌘ and scroll, pinch or the buttons to zoom; hover for the values under the pointer; reset returns to the region.

Environmental exposure, framed on Africa.Satellite and model products as NASA publishes them, each with its own date. They are indicators, not assessments of any place.

Data: NASA GIBS · ASTER GDEM · Blue Marble · © OpenStreetMap contributors · Natural Earth · NASA EONET · ESA Copernicus Sentinel-5P TROPOMI · NASA JPL OPERA DIST-ALERTSources and licences

  • We acknowledge the use of imagery provided by services from NASA’s Global Imagery Browse Services (GIBS), part of NASA’s Earth Science Data and Information System (ESDIS). Colours redrawn from the published colour map; values averaged into hexagonal cells where shown (modified). NASA open data
  • We acknowledge the use of imagery provided by services from NASA’s Global Imagery Browse Services (GIBS), part of NASA’s Earth Science Data and Information System (ESDIS). Colours redrawn from the published colour map; values averaged into hexagonal cells where shown (modified). NASA open data
  • Center for International Earth Science Information Network (CIESIN), Columbia University. 2018. Gridded Population of the World, Version 4 (GPWv4): Population Density, Revision 11. NASA Socioeconomic Data and Applications Center (SEDAC). Licensed under CC BY 4.0. Colours redrawn from the published colour map; values averaged into hexagonal cells where shown (modified). doi:10.7927/H49C6VHW. CC BY 4.0
  • VIIRS near-real-time products (flood, land surface temperature, snow cover, corrected reflectance), NASA LANCE; imagery via NASA GIBS. Colours redrawn from the published colour map; values averaged into hexagonal cells where shown (modified). NASA open data
  • MODIS and VIIRS atmosphere products, NASA LANCE and LAADS DAAC; imagery via NASA GIBS. Colours redrawn from the published colour map; values averaged into hexagonal cells where shown (modified). NASA open data
  • OMI nitrogen dioxide (Aura) and OMPS aerosol index (Suomi NPP), NASA; distributed by the NASA GES DISC; imagery via NASA GIBS. Colours redrawn from the published colour map; values averaged into hexagonal cells where shown (modified). NASA open data
  • MODIS land products (vegetation indices, gross primary production, land cover), NASA LP DAAC and LANCE; imagery via NASA GIBS. Colours redrawn from the published colour map; values averaged into hexagonal cells where shown (modified). NASA open data
  • ASTER GDEM is a product of METI and NASA (ASTER Global Digital Elevation Model V003, NASA LP DAAC); imagery via NASA GIBS. Colours redrawn from the published colour map; values averaged into hexagonal cells where shown (modified). Cite as: NASA/METI/AIST/Japan Spacesystems and U.S./Japan ASTER Science Team (2019). ASTER Global Digital Elevation Model V003. doi:10.5067/ASTER/ASTGTM.003. NASA open data
  • Made with Natural Earth. Free vector and raster map data @ naturalearthdata.com. Public domain
  • © OpenStreetMap contributors (ODbL), via NASA GIBS (coastlines, boundaries, roads and place names). Colours redrawn from the published colour map; values averaged into hexagonal cells where shown (modified). ODbL
  • Blue Marble, NASA Earth Observatory; imagery via NASA GIBS. Colours redrawn from the published colour map; values averaged into hexagonal cells where shown (modified). NASA open data
  • Context on the globe and flat map: coastlines, boundaries, roads and place names from NASA GIBS reference layers (OpenStreetMap data, ODbL, © OpenStreetMap contributors); Blue Marble imagery as an optional base; 3D terrain and the elevation read-out from the ASTER GDEM elevation colour index (5 m to 200 m classes, exaggerated for legibility). Boundaries are drawn as published and imply no position on any territory.
  • Live events: NASA Earth Observatory Natural Event Tracker (EONET); events are indicative and keep their original source links.
  • Live: NASA EONET Events are indicative, not authority warnings.
  • Observed (EO): ESA Copernicus Sentinel-5P TROPOMI (contains modified Copernicus Sentinel data 2026). We acknowledge the use of imagery provided by services from NASA’s Global Imagery Browse Services (GIBS), part of NASA’s Earth Science Data and Information System (ESDIS).
  • Observed (EO): NASA JPL OPERA DIST-ALERT (contains modified Copernicus Sentinel data 2026). We acknowledge the use of imagery provided by services from NASA’s Global Imagery Browse Services (GIBS), part of NASA’s Earth Science Data and Information System (ESDIS).
  • NASA GIBS: We acknowledge the use of imagery provided by services from NASA’s Global Imagery Browse Services (GIBS), part of NASA’s Earth Science Data and Information System (ESDIS).
  • Air quality forecast, 5 days, PM2.5 (CAMS via Open-Meteo): Contains modified Copernicus Atmosphere Monitoring Service information 2026.
  • Past year, daily temperature (ERA5 via Open-Meteo): Contains modified Copernicus Climate Change Service information 2026.
  • Weather data by Open-Meteo.com (https://open-meteo.com/), licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/).
  • Optional layers (14 sources in the layer list: live, forecast, observed and context) print their provider credit on the map and here when switched on; forecasts are model output and derived indices are estimates.

Layers and dates

  • Population density, 2020: Population: CIESIN, Columbia University, Gridded Population of the World v4.11 (2018), NASA SEDAC, CC BY 4.0 · static
  • Land surface temperature, day, 8-day composite: VIIRS NOAA-20 Level 3, 8-day, NASA LANCE · 14 Sep 2026
  • Aerosol optical depth, daily: VIIRS NOAA-20 Deep Blue AOD, NASA LANCE · 13 Oct 2026
  • True colour: VIIRS NOAA-20 corrected reflectance, NASA LANCE · 13 Oct 2026
  • Nitrogen dioxide, lower atmosphere column: OMI Aura, NASA GES DISC · 10 Oct 2026
  • Land cover type (IGBP), yearly: MODIS Terra and Aqua land cover (MCD12Q1), NASA LP DAAC · 2024

04Couplings

How epidemics travel through water, food, energy and nature.

DomainHow it couples hereWhat to watch
WaterCholera and other waterborne diseases follow failures of water and sanitation. See drinking water and sanitation.Outbreaks after floods and supply failures
FoodFood-borne outbreaks follow cold chain and hygiene failures; lockdowns disrupt food access.Food-borne outbreak investigations
EnergyVaccine cold chains and hospital surge capacity need reliable power.Cold chain equipment functionality
BiodiversityVector ranges shift with climate and land use. See ecosystem integrity.Vector surveillance at range margins

The legal frame changed in 2024 and 2025. Amendments to the International Health Regulations adopted by the World Health Assembly in June 2024, in force from September 2025, introduced a pandemic emergency alert level. The WHO Pandemic Agreement was adopted in May 2025; its annex on pathogen access and benefit-sharing is under negotiation and the agreement opens for signature only after that annex is adopted.

Transmission is described by the basic and effective reproduction numbers (dimensionless), the serial interval in days and the case fatality ratio, each estimated with uncertainty that is largest early in an outbreak.

05Monitor

Surveillance and capacity board.

The screen a team would use to follow this subject: its measures, states and decisions. Names are examples and values are illustrative.

Surveillance and capacity board: the screen’s structure, terms and states for Infectious threats.Example names and illustrative values; a design, not a record or a live system.
06Reports, standards and contributions

Infectious threat evidence and its boundaries.

Preparedness and modelling studies are designed for the Scenario and Stress-Test series. The Global Centre for Risk and Innovation (GCRI) declares no emergency and issues no health advice; WHO and national public health authorities hold those mandates. Epidemic models would carry NS-12 cards under the proposed Nexus Standards. Useful contributions: retrospective model evaluations and preparedness exercise records.

07Simulator

SEIR on a network of places.

An outbreak seeded in one place spreads through a small-world network by the share of contacts people make elsewhere; play it to see the order of arrival and how the timing of measures moves the peak. Teaching model with stated illustrative values, not a prediction. The map on this page shows Africa; the model itself is not tied to a place.

Controls

Basic reproduction number R0
2.5
Latent period
3.0 days
Infectious period
5.0 days
Share of contacts made in other places
8%
Places in the network
24
Measures start on day
40
Contact reduction from measures
40%
Days simulated
240 days

Readouts

Peak new infections
6.20 per 1,000 a day
Day of the peak
158
Share ever infected
58%
Places reached
100%
Reproduction number under measures
1.50
Numbers, methods and sources
DayNew infections per 1,000Cumulative infected (%)
00.000.0
170.020.0
340.160.1
510.470.7
680.851.8
851.443.8
1022.397.0
1193.7012.2
1365.1319.8
1536.1329.6
1705.8239.9
1874.1548.4
2042.2953.8
2211.0756.5
2380.4657.7

Methods

  1. SEIR compartments Kermack and McKendrick (1927), Proc. R. Soc. A 115
  2. Metapopulation coupling by mobility Balcan and others (2009), PNAS 106
  3. Small-world network Watts and Strogatz (1998), Nature 393
  4. Modelling infectious diseases Keeling and Rohani (2008), Princeton University Press
  5. Uncertainty fan (parameter Monte Carlo) Spiegelhalter, Pearson and Short (2011), Visualising uncertainty about the future, Science 333
  6. One-at-a-time sensitivity (tornado) Saltelli and others (2008), Global Sensitivity Analysis: The Primer, Wiley

Data

The model runs on the teaching values set by the controls. To calibrate it for a place, read it against these admitted layers and series, which this page’s map can draw:

  • Cities Natural Earth
  • Airports Natural Earth
  • Major roads Natural Earth
SEIR on a network of places. Illustrative teaching model: it runs on the values set by the controls, not on measured data. Methods: SEIR compartments (Kermack and McKendrick (1927), Proc. R. Soc. A 115); Metapopulation coupling by mobility (Balcan and others (2009), PNAS 106); Small-world network (Watts and Strogatz (1998), Nature 393).

Across the six pillars, led by Nexus Consortium

08Indicators

What is measured, and how well.

Each indicator carries its unit, its cadence, the kind of source behind it and the evidence label a value would carry. No values are shown: a programme supplies them under its own authority.

IndicatorUnitCadenceSource typeLabel
Notified cases by date of onsetcasesdaily or weeklyIndicator-based surveillanceREPORTED
Time-varying reproduction numberdimensionlessweeklyTransmission models on onset and case dataESTIMATED
Test positivitypercent of tests positiveweeklyLaboratory reportingOBSERVED
Sequencing coveragepercent of confirmed cases sequencedweeklyGenomic surveillance and public repositoriesOBSERVED
Timeliness of detection, notification and responsedays between emergence, detection, notification and responseper eventEvent records against national timeliness targetsREPORTED
Influenza-like illness and severe acute respiratory infectionconsultations or admissions per hundred thousand populationweeklySentinel surveillance in GISRSOBSERVED
Case fatality riskpercent of confirmed cases with known outcomeper eventLine lists with outcome follow-upESTIMATED
Excess mortalitydeaths above expectedweekly or monthlyVital registration against modelled baselinesESTIMATED
09Twin and cascade simulator

Model infectious threats as a system. Run the cascade.

Assembled from the Nexus Standards area profile Health security and One Health: the hazards it names, the systems and sectors that carry the loss, the controls that dampen it and the data that observe it. Choose the components, pick an initiating hazard and step through the cascade. The Map view places the run on an example geography with open data. Every coupling weight is a stated assumption you can change. A teaching model, not a forecast, an assessment or a certification. To model a place of your own choosing, open the full System Builder.

Hazards

Controls

  • NS-0 Constitution and principles
  • NS-3 Risk record UNIs (grammar, legacy crosswalk, allocation authority)
  • NS-9 Contextual UNIs (deployments, instances, programme instances)
  • NS-10 Policies and clauses (machine-readable obligations: definition, binding, evaluation and enforcement)
  • NS-11 Verification and monitoring (evidence, attestations, zero-trust conformance observation)
  • NRS-H83 Hazard family: Health system capacity failure (NHS-1 health.system)
  • NRS-H42 Hazard family: Rare and persistent human infections (NHS-1 health.other-human)
  • NRS-H41 Hazard family: Animal-to-human spillover (NHS-1 health.zoonotic)
  • NRS-H43 Hazard family: Livestock and aquaculture epizootics (NHS-1 health.animal-health)
  • NRS-D3 Humanitarian response and resilient recovery
  • NRS-H46 Hazard family: Environmental and product harm to people (NHS-1 health.harm)

Observation

  • WHO Disease Outbreak News (listed for reference)
  • WAHIS animal health events (listed for reference)
  • WorldPop gridded population (listed for reference)
  • International Health Regulations capacity and health emergency preparedness
  • Percentage of bloodstream infections due to selected antimicrobial-resistant organisms

Example geography

  • West Africa: 59 hexagonal cells
  • Night lights tonight, outages (VIIRS Day/Night Band), NASA GIBS
  • Surface water through cloud, 30 m (OPERA DSWx-S1 radar), NASA GIBS
  • Rain rate, last half hour (IMERG), NASA GIBS
10Sources

Where the evidence comes from.

Named public datasets and institutional records, each with its licence class. Sources by agreement are used only under their terms and never republished.

SourceWhat it givesLicence class
WHO Disease Outbreak NewsPublic reports on acute public health eventsPublic records, WHO terms of use
WHO GISRS: FluNet and FluIDVirological and epidemiological influenza surveillance from national influenza centresOpen, attribution, WHO terms of use
GISAIDInfluenza and coronavirus genomes with sampling metadataRegistered access under the GISAID database access agreement
INSDC sequence archives and NextstrainOpen pathogen sequences and phylogenetic analysesFree and unrestricted; open, attribution
ECDC Surveillance Atlas and Africa CDC epidemic intelligence reportsRegional surveillance data and weekly event reportsOpen, attribution, terms per agency
ProMED and HealthMapEvent-based signals from media, professional networks and informal reportsTerms per service
Global.healthDe-identified case line lists for selected outbreaksOpen, attribution (CC BY 4.0)
11Methods and evidence

How infectious threats is modelled, measured and tested.

The model, its assumptions, its data and its uncertainty, in the form a scientist or engineer would check. Structures follow published methods. The values drawn are illustrative, not findings.

System model

The couplings between the systems in infectious threats: signs of influence, a reinforcing and a balancing loop, and cross-links where stress jumps.

++−HealthCommunitiesEconomyR reinforcingB balancing
Causal loop diagram from the page’s systems; signs show the direction of influence. Illustrative structure for a system dynamics or network model, not estimated couplings.
FAIR guiding principles · GO FAIR and FORCE11W3C PROV · World Wide Web ConsortiumGuidance on consistent treatment of uncertainties · IPCCCEOS Analysis Ready Data · Committee on Earth Observation Satellites
12Decisions

Who decides, and what the evidence serves.

Health Nexus prepares evidence. The decisions below are taken by the authorities named, under their own rules, and never by the platform.

Who decidesThe questionEvidence this page serves
National public health institutesWhether a signal is a true event, and what investigation and response it warrantsSignal verification trail; line lists; nowcasts with their uncertainty
IHR National Focal PointsWhether an event meets the criteria for notification to WHO under the IHR decision instrumentEvent assessments with the evidence for each criterion labelled
Hospital systemsHow much surge capacity to activate, and whenAdmission scenarios by region with confidence bands
Ministries of health and cabinetsWhich public health and social measures to apply, adjust or lift, and for how longTransmission estimates; evidence on the effects of measures with its limits; cascade scenarios

Outputs this topic produces

  • Outbreak situation evidence with reporting delays and completeness stated
  • Nowcasts and short-term scenarios with their skill reported
  • Genomic context for emerging variants and lineages
  • Readiness evidence read against IHR core capacities
13Core questions

What experts ask.

Most decisions about outbreaks are made before the facts are settled.

  • Is an unusual signal a real outbreak, and how fast is it growing?
  • How transmissible and how severe is the pathogen, and for whom?
  • How much transmission occurs before symptoms appear?
  • What effect would testing, isolation, vaccination or other measures have, and at what social cost?
  • When does health-system demand exceed capacity?
14Mechanisms

Mechanisms and drivers.

Transmission depends on the pathogen’s biology, contact patterns between people, immunity in the population and the environment. The reproduction number summarises how many people each case infects on average; when it stays above one, case numbers grow. Mobility, crowding, seasonality and human behaviour change the reproduction number over time.

Severity depends on age, underlying conditions, access to care and how stretched the health system is. A pathogen of moderate severity can still overwhelm hospitals if it spreads widely, which is why the indirect harm from disrupted services can rival the direct harm of the infection.

15Methods and data

How it is studied, and where the methods stop.

Method or dataWhat it showsLimits
Indicator and event-based surveillanceUnusual clusters and trends from reports, laboratories and mediaReporting delays, under-ascertainment and changing testing practice
Genomic surveillanceVariants, transmission chains and introductionsUneven sequencing capacity and sampling bias
Compartmental and agent-based modelsScenarios of spread and the effect of interventionsHighly sensitive to uncertain parameters early in an outbreak
Nowcasting and short-term forecastingCurrent and near-future trends corrected for reporting delayReliable only over short horizons
Serological surveysHow many people have been infectedAntibody waning and test performance complicate interpretation
18Questions

Questions and answers.

Why do early outbreak numbers change so much?
Because reports arrive late, mild cases are missed and outcomes are not yet known. Estimates are revised as data mature, and nowcasting methods adjust for the delays that can be measured.
What is the difference between a forecast and a scenario?
A forecast states what is expected to happen with a probability and can be checked against what happens. A scenario explores what would follow from stated assumptions, such as a change in measures, and is not a prediction.
What is a public health emergency of international concern?
Under the International Health Regulations, an extraordinary event that constitutes a public health risk to other states through international spread and may require a coordinated international response. The WHO Director-General determines it, after advice from an emergency committee.
Does Health Nexus issue outbreak alerts?
No. Alerts and notifications come from national public health authorities, IHR National Focal Points and WHO. Health Nexus prepares evidence those bodies can examine under their own procedures.
Are outbreak models predictions?
Models are usually scenarios: they show what could happen under stated assumptions. Short-term forecasts are labelled as such and carry uncertainty ranges. Neither is a statement of what will happen.
Why do early estimates change so much?
Early data is sparse, delayed and biased towards severe cases. As testing expands and studies report, estimates of transmissibility and severity are revised, which is expected and should be communicated openly.
Does Health Nexus publish outbreak alerts?
No. Alerts and public health guidance come from competent health authorities. Health Nexus publishes reviewed research and methods.
ReportsContribute

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Reports is the Consortium’s quarterly publication. What you know about this subject can become evidence, a case or a correction in the next edition, credited to you and published under CC BY 4.0 where it is your own work.

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