The factor most likely to cause an outbreak of a disease spread from person to person to become an epidemic is a high population density. When people live in close proximity to each other, there is a greater likelihood of the disease spreading rapidly from person to person. This is especially true in crowded urban areas, refugee camps, or areas with poor sanitation and healthcare infrastructure. Other factors that can contribute to the spread of a disease and the development of an epidemic include lack of access to healthcare, poor hygiene practices, and a lack of widespread vaccination coverage.
