Late Mary Boyce attributes lower outbreaks of pandemics among Zoroastrian communities to the strict purity laws and practices. For example, it was unacceptable to pollute water, considered one of the sacred creations of Ahuramazda. To avoid polluting water, Zoroastrians would not wash their hands directly in rivers or bodies of water. There are hundreds of examples of these strict purity laws. [1]

[1] A History of Zoroastrianism, The Early Period by Mary Boyce <https://archive.org/details/AHistoryOfZoroastrianismVolI>