The answer is: Touching dead bodies

Lev 21:1 - Priests had to maintain ceremonial cleanliness to perform their duties. An exception was made for close relatives.

In the book of Leviticus, the rules laid out for priests may seem strict, but they reflect deeply rooted cultural values in ancient Israelite society, emphasizing holiness and separation from impurity. Priests were essentially the middlemen between God and the people. Their conduct not only affected their ability to perform rituals but also shaped the community's relationship with the divine.

Touching a dead body was considered a significant defilement because death symbolized the ultimate consequence of sin. Interestingly, this concern wasn’t just a priestly issue; it pervaded the entire community. Cleanliness laws extended to all Israelites, showing that, while the priests had specific roles, everyone was called to a standard of purity.

Moreover, they could break this rule when mourning close relatives—these possible exceptions highlight the importance of familial bonds even within the context of religious duties. This is echoed in what Jesus later taught about prioritizing love, compassion, and connection with family over stringent ritualism.

Did you know that the Hebrew word for "holiness," *qodesh,* literally means "to separate"? This theme of separation—from the common to the sacred, from life to ritual duty—resonates throughout the scriptures, representing a longing for a world where life, love, and reverence for the divine harmoniously exist. It’s all about the connection: suspicion of death and separation expresses a profound yearning for life, holiness, and relationships that foster spiritual growth.