Some of these diseases, such as cholera and measles, spread rapidly in large populations living in close proximity, such as in a refugee camp, and people must be immunised by the hundreds or thousands in a short period of time.
Nogle af disse sygdomme, som kolera og mæslinger, spredes hurtigt i store populationer der lever tæt sammen, som i flygtningelejre, hvorfor der kan være behov for at vaccinere folk i hundredvis eller endog tusindvis på kort tid.