Charak Samhita
तमुवाच भगवानात्रेयः- एवमसामान्यावतामप्येभिरग्निवेश! प्रकृत्यादिभिर्भावैर्मनुष्याणां येऽन्ये वाः सामान्यास्तद्वैगुण्यात् समानकालाः समानलिङ्गाश्च व्याधयोऽभिनिर्वर्तमाना जनपदमुद्ध्वंसयन्ति। ते तु खल्विमे भावाः सामान्या जनपदेषु भवन्ति; तद्यथा- वायुः, उदकं, देशः,काल इति ॥६॥
[Atreya’s explanation] Lord Atreya replied “Agnivesha! Though a community may be dissimilar with respect to the constitution and other characteristics of its individuals, there are other common factors which often get adversely affected, resulting in simultaneous outbreak of diseases having similar symptoms that destroy communities. The common factors which affect mass population are vayu (air), jala (water), desha (land) and kala (season).
english translation