How was the satellite habitat data collected?

We queried Sentinel-2 imagery through Google Earth Engine for three endemic regions and computed region-averaged NDVI and peak MNDWI. The Habitat Suitability Index (0–10) weights vegetation density and peak water presence equally. It is our own composite indicator of breeding conditions — a comparison aid, not a direct mosquito count or an official epidemiological measure.

How many mosquito bites does it actually take to get yellow fever?

There is no single number — it depends on how many mosquitoes are infectious. Using the compound Bernoulli model P(n) = 1 − (1 − f·p)ⁿ with field-measured infection rates: at the inter-epidemic baseline (~0.2%), about 578 bites for a 50% cumulative chance; during an active outbreak (~5%), that falls to about 23 — reachable in one evening in an endemic zone. These are modelled orders of magnitude, not clinical guarantees.

Is yellow fever a risk in Southeast Asia or Taiwan?

No — there has never been a recorded outbreak in Asia. The puzzle is that there arguably should be: Aedes aegypti is widespread, and laboratory studies show Asian populations (including in Taiwan and Singapore) can transmit the virus, so vector competence is not the barrier. Why Asia has stayed free of yellow fever remains genuinely unresolved; leading hypotheses include cross-protection from the region's heavy dengue exposure and historical patterns of spread. Vaccination is still required to enter some countries when arriving from an endemic region.

Why does the Amazon score highest on habitat if Africa reports more outbreaks?

Habitat suitability measures conditions for breeding — and the Amazon's contiguous forest and river systems score exceptionally high. Outbreaks, though, also need susceptible people: they turn on population density, vaccination coverage, surveillance and vector control. West Africa's large unvaccinated urban populations create amplification that the sparsely populated Amazon does not, and reported frequency also reflects differences in surveillance. Suitability is potential energy; an outbreak is the spark plus the fuel of susceptible hosts.

FIELD NOTES2026-06-15 · 10 min read

The Bite Count

How many mosquito bites does it take to get yellow fever? Real Sentinel-2 satellite data from three endemic zones — and the probability model that puts a number on the risk.

Illustration: a near-infrared false-colour rendering of an Amazon floodplain — forest canopy reflects infrared as crimson, standing water reads blue-black. Indicative, not a measurement.

The mosquito is the deadliest animal to humans — not through its own venom, but as a vector for the pathogens it carries, which kill an estimated 725,000 to 1,000,000 people every year, more than any other creature. Yellow fever is one of its most lethal cargoes: a modelling study estimated 84,000–170,000 severe cases and 29,000–60,000 deaths in Africa in a single year, roughly 90% of the global burden (Garske et al., 2014). What makes yellow fever unusual is that we have a near-perfect vaccine, a well-characterised vector, and reliable satellite proxies for its habitat. That combination makes it possible to do something rare in infectious disease: attach a probability to the risk.

What the satellite measures

Two Sentinel-2 indices proxy mosquito habitat from orbit. NDVI — the Normalized Difference Vegetation Index — measures living green cover: denser vegetation gives adult mosquitoes resting sites and holds the humidity that larvae need. MNDWI — the Modified Normalized Difference Water Index — detects surface water: the rivers, pools and flooded ground where Aedes breeds. We queried real Sentinel-2 imagery through Google Earth Engine for three endemic zones and derived a composite Habitat Suitability Index (HSI), scored 0–10, weighting vegetation density and peak water presence equally. The HSI is our own construct — a way to compress two satellite signals into one comparable number, not an established epidemiological metric.

Three zones, measured from orbit

West Africa — Nigeria/Benin axis (2.5–5.5°E, 6.2–8.5°N): NDVI mean 0.517, MNDWI peak 0.522, HSI 6.4/10. This corridor runs from the Niger Delta wetlands to the Guinea forest edge — a mosaic of vegetation and seasonal standing water carrying Aedes aegypti alongside the sylvatic vectors Ae. africanus and Ae. luteocephalus. It is also where the hard field numbers come from: Nigeria's nationwide surveillance during the 2017–2020 outbreaks detected yellow fever virus in about a quarter of Aedes mosquito pools, with minimum infection rates ranging from 0.9 to 62.5 per 1,000 across species (Parasites & Vectors, 2024) — the empirical anchor for the model below.

West Africa — Nigeria/Benin axis (2.5–5.5°E). Sentinel-2 near-infrared false-colour, dry-season composite 2024–25. Vegetation reflects infrared as red; the Gulf of Guinea coast and lagoons read dark. HSI 6.4/10.

Amazon Basin — Manaus area, Brazil (61–58°W, 4–1°S): NDVI mean 0.637, MNDWI peak 1.0, HSI 8.2/10 — the highest of the three. NDVI above 0.6 indicates near-continuous primary canopy; an MNDWI peak of 1.0 marks open water — the floodplain lakes and tributary channels that braid hundreds of kilometres of forest-water edge. This is ideal habitat: the sylvatic yellow-fever cycle here runs through Haemagogus and Sabethes mosquitoes biting by day in the canopy, with Aedes aegypti carrying the urban cycle in settlements. Suitability is highest precisely where forest meets standing water.

Amazon Basin — Manaus area (61–58°W). Sentinel-2 near-infrared false-colour, 2024–25 composite. Dense canopy in deep red; river channels and flooded forest in blue-black. HSI 8.2/10.

Congo Basin — Kinshasa area, DRC (14.5–17.5°E, 5–2°S): NDVI mean 0.504, MNDWI peak 0.827, HSI 7.1/10. Dense equatorial forest threaded by the Congo River and its tributaries. It is also one of the least entomologically surveyed endemic zones — a real intelligence gap for a region that records yellow fever every year. The satellite read matches West Africa on overall suitability, with lower spatial variability: more continuous forest, less fragmentation.

Congo Basin — Kinshasa area (14.5–17.5°E). Sentinel-2 near-infrared false-colour, 2024–25 composite. The Congo River and a crater lake stand out against continuous canopy. HSI 7.1/10.

The probability model

Bites and infection follow a compound Bernoulli process. Let f = the fraction of biting Aedes carrying viable yellow fever virus, and p = the probability that one infectious bite transmits. The chance of at least one infection after n independent bites is P(n) = 1 − (1 − f·p)ⁿ. The model is deliberately simple: it treats each bite as an independent trial at constant risk and ignores acquired immunity, so read the numbers as orders of magnitude, not clinical predictions. Its value is that every input is stated and swappable.

Inter-epidemic baseline: take f ≈ 0.002 — the low end of the measured Nigerian range — and p ≈ 0.6, mid-range for Aedes vector competence in the laboratory, giving an effective per-bite risk f·p ≈ 0.0012. The model then yields 88 bites for a 10% cumulative chance of infection, 578 bites for 50%, and 1,917 bites for 90%. In a rural West African endemic area at peak season, where biting can reach tens per night, 578 bites accumulate in roughly two to three weeks of unprotected outdoor exposure.

The model, plotted. Amber: outbreak (f·p ≈ 0.03). Forest green: endemic (f·p ≈ 0.0012). The 50% thresholds fall at 23 and 578 bites.

Under the same model, a confirmed outbreak collapses the count to 23 bites for a 50% chance of infection — a total an unprotected adult can reach in a single evening outdoors at the peak of the season.

Outbreak phase: vector infection prevalence climbs. Nigeria's own surveillance recorded minimum infection rates up to 62.5 per 1,000 (≈6%) in the most-infected species, so an outbreak f ≈ 0.05 is conservative; with p ≈ 0.6 that gives f·p ≈ 0.03. Now the model returns 4 bites for 10%, 23 bites for 50%, and 76 bites for 90%. These are not abstractions: across the Americas in 2025, PAHO confirmed 346 yellow-fever cases and 143 deaths in seven countries — a 41% case-fatality rate — with the virus reaching São Paulo state and other areas outside the historical risk map.

The disease that earns its name

Yellow fever is named for the jaundice of its severe form. About 85% of symptomatic infections resolve after the acute phase — sudden fever, severe headache, myalgia, and Faget's sign, the paradoxical slowing of the pulse as fever climbs. After a brief remission, roughly 15% enter the toxic phase: jaundice, haemorrhage (the 'vomito negro', black vomit of degraded blood), simultaneous liver and kidney failure, and shock. WHO estimates that about half of those who enter the toxic phase die within 7 to 10 days. There is no antiviral; care is supportive. Reported case fatality rises steeply when outbreaks are caught late — 41% across the Americas in 2025 — almost entirely among the unvaccinated.

The only number that resets the probability to zero

The 17D yellow-fever vaccine reaches ≥99% seroconversion within 30 days of a single dose, and in 2013 WHO confirmed that protection is lifelong — no booster required. Every strand above converges on one line: vaccinate before entering high-HSI territory. The mosquito does not keep your calendar; the virus does not read your budget. The satellite only shows where both of them live. Intelligence, harvested.

Frequently asked

How was the satellite habitat data collected?: We queried Sentinel-2 imagery through Google Earth Engine for three endemic regions and computed region-averaged NDVI and peak MNDWI. The Habitat Suitability Index (0–10) weights vegetation density and peak water presence equally. It is our own composite indicator of breeding conditions — a comparison aid, not a direct mosquito count or an official epidemiological measure.
How many mosquito bites does it actually take to get yellow fever?: There is no single number — it depends on how many mosquitoes are infectious. Using the compound Bernoulli model P(n) = 1 − (1 − f·p)ⁿ with field-measured infection rates: at the inter-epidemic baseline (~0.2%), about 578 bites for a 50% cumulative chance; during an active outbreak (~5%), that falls to about 23 — reachable in one evening in an endemic zone. These are modelled orders of magnitude, not clinical guarantees.
Is yellow fever a risk in Southeast Asia or Taiwan?: No — there has never been a recorded outbreak in Asia. The puzzle is that there arguably should be: Aedes aegypti is widespread, and laboratory studies show Asian populations (including in Taiwan and Singapore) can transmit the virus, so vector competence is not the barrier. Why Asia has stayed free of yellow fever remains genuinely unresolved; leading hypotheses include cross-protection from the region's heavy dengue exposure and historical patterns of spread. Vaccination is still required to enter some countries when arriving from an endemic region.
Why does the Amazon score highest on habitat if Africa reports more outbreaks?: Habitat suitability measures conditions for breeding — and the Amazon's contiguous forest and river systems score exceptionally high. Outbreaks, though, also need susceptible people: they turn on population density, vaccination coverage, surveillance and vector control. West Africa's large unvaccinated urban populations create amplification that the sparsely populated Amazon does not, and reported frequency also reflects differences in surveillance. Suitability is potential energy; an outbreak is the spark plus the fuel of susceptible hosts.