Key points
- WHO names conflict, displacement, climate events and lack of investment in water and sanitation as drivers of cholera — the same forces recur across outbreaks.
- Vaccination gaps are a major driver of measles: global coverage was 84% first dose and 77% second dose in 2025, against 95% needed.
- Climate and travel are expanding the range of Aedes mosquitoes, bringing dengue and chikungunya to mainland Europe.
- Every driver on this page is one an official source states. No single cause is blamed for a multi-cause outbreak.
Direct answer: Outbreaks happen for reasons official sources can name: conflict and displacement, unsafe water and sanitation, gaps in vaccination, a changing climate, travel and cross-border movement, and strained health systems and funding. No outbreak has a single cause. This page takes each driver in turn, with the official source that states it.
Conflict and displacement
Conflict is the driver WHO names first for cholera, and it recurs across diseases. WHO’s cholera fact sheet says outbreaks “may be due to conflict, population displacement, climate events like cyclones, floods or drought, and lack of investment in maintaining and improving WASH services”. Yemen is the sharpest example: escalating conflict from 6 August 2026 displaced more than 200,000 people across 10 governorates into overcrowded camps with unsafe water, and cholera cases tripled in three weeks (WHO EMRO, 13 October 2026). In Sudan, WHO attributes the severity of cholera to the war, 13.4 million displaced people, overwhelmed facilities and a supply blockade from December 2024 to July 2026. Displacement does the damage in a specific way: it crowds people together, breaks water and sanitation, interrupts vaccination, and overwhelms the clinics that would catch an outbreak early.
Unsafe water, sanitation and hygiene
Cholera is the clearest case: “Access to safe water, basic sanitation and hygiene (WASH) is essential to prevent cholera”, and cholera “indicates inequity and lack of social and economic development” (WHO cholera fact sheet). Where water is unsafe and sanitation is absent, water-borne disease has a permanent route into a population. The same failure underlies the 2.1 billion people without safely managed drinking water and the 3.4 billion without safely managed sanitation recorded by the WHO/UNICEF Joint Monitoring Programme. The WASH guide on this site explains the service ladder behind those numbers.
Vaccination coverage gaps
Measles is the textbook case: “Gaps in vaccination coverage are a major driver” of its return. Global first-dose measles coverage was 84% and second-dose 77% in 2025, against the 95% with two doses needed in every community to stop transmission (WHO/UNICEF, 15 July 2026). In the United States, kindergarten MMR coverage fell from 95.2% in 2019–20 to 92.4% in 2025–26 (CDC). Yellow fever shows the same pattern: WHO attributes recent activity in the African Region to uneven vaccination coverage and immunity gaps in districts where preventive vaccination was not implemented. Polio persists for the same reason — about 250,000 children in South Khyber Pakhtunkhwa were unreached, and no house-to-house campaigns have run in Afghanistan since October 2024 (45th Polio Emergency Committee, 25 August 2026).
Climate, weather and mosquito range
Dengue and chikungunya spread because the mosquitoes that carry them are spreading. WHO’s chikungunya risk assessment (24 April 2026) names “climate- and conveyance-driven mosquito range expansion” and rainy seasons creating Aedes breeding conditions; Aedes albopictus is now established in 16 EU/EEA countries, and locally acquired dengue and chikungunya now occur in mainland Europe. Climate also drives cholera: WHO has warned that the rainy season could accelerate spread in the DRC where sanitation is limited, and South Sudan’s WHO El Niño health plan (1 November 2026 – 30 April 2027) pre-positions cholera kits ahead of floods, drought and heat.
Travel, cross-border movement and urbanisation
People move, and germs move with them. Yellow fever risk rises with “frequent cross-border movement” (WHO); polio’s committee cites cross-border movement and genetic evidence of cross-border transmission between Afghanistan and Pakistan in early 2026; CDC notes that rising global measles activity means “more chances of an unvaccinated person infected with measles abroad returning” home; and Ebola’s imported cases reached Kenya and Nigeria by road and air travel. Urbanisation adds density: crowded cities with weak water and sanitation give an outbreak more people to reach, faster.
Funding and health-system strain
The last driver is the quietest: money and systems. WHO’s 2026 appeal asks for $1 billion for the 36 most-severe emergencies, after only $900 million of $1.5 billion arrived in 2025, and WHO’s health-emergencies chief has warned that funding cuts leave the world “more vulnerable to epidemics and pandemics”. Yemen’s 90-day cholera response had only about $4.3 million of $31 million secured. The Global Polio Eradication Initiative faces a 30% budget cut in 2026 and a $1.7 billion funding gap. These figures are press reporting of WHO remarks and should be re-sourced to who.int before reuse — but the mechanism is not in doubt: when surveillance, vaccination and water systems lose funding, outbreaks get more room.
Frequently asked questions
Is there one main cause of outbreaks?
No. Outbreaks have several drivers at once — conflict, water and sanitation, vaccine gaps, climate, travel and funding. This page lists only the drivers official sources state, and does not blame a single cause.
Why does conflict cause disease outbreaks?
It displaces people into crowded camps with unsafe water, breaks health services and supply lines, and stops vaccination campaigns — the exact conditions cholera, measles and polio exploit.
How does climate change affect outbreaks?
It expands the range of mosquitoes that carry dengue and chikungunya, and floods and droughts disrupt water and sanitation, which drives cholera and other water-borne disease.