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Why does disease spread more quickly in densely packed populations?

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Common wildebeest — population ecology A herd of common wildebeest (*Connochaetes taurinus*) migrating across a dusty savanna in Africa. The animal is a keystone species (i.e., a species with a disproportionately large effect on its biological community) in plains and acacia savanna ecosystems from southeastern Africa to central Kenya.

In population ecology, disease transmission is considered a density-dependent factor, meaning its influence on a population changes based on the number of individuals per unit area.1

Disease spreads more quickly in densely packed populations primarily because individuals live in closer proximity to one another, which increases the frequency of contact and facilitates the transmission of infectious agents. When populations become so dense that they result in overcrowding, living conditions and sanitation often decline, further accelerating the rate at which agents are transmitted.12

In these environments, high density can create ideal conditions for infectious agents to thrive and potentially evolve. Conversely, as population density decreases—such as through mortality or migration—the frequency of contact between individuals drops, which typically leads to a decline in disease transmission.21