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.
Vissa av dessa sjukdomar, som kolera och mässling, sprids fort i stora populationer som lever mycket tätt, såsom i ett flyktingläger, och människor måste immuniseras i hundratals eller tusentals inom en kort tidsrymd.