Ki Tisa(Exodus 30:11-34:35)
During the Middle Ages, it was not uncommon for Jews to be accused of poisoning wells and infecting the gentile populace with plagues. Lending support to their claim, the Jews' opponents would point to the fact that Jewish rates of illness was typically lower than the general population.
This statistic may well have been true. Jews have always been known for their concern with cleanliness and better sanitation. Jewish tradition particularly stood out during the Middle Ages - when royalty would often put on perfume rather than wash, and where in colder climates it was not uncommon for people to sew themselves into their garments for an entire winter.
There are many Torah sources which speak about proper hygiene. The Talmud, written 1600 years ago, reports how the great sage Hillel emphasized to his students the importance of bathing regularly. Jewish law also requires that one bathe as a part of Shabbos preparations.
In addition, there are many Jewish rituals that require washing for "spiritual" reasons: e.g. immersion in the Mikveh, and washing hands before prayer, before eating bread, and when getting up in the morning.
One of the first explicit associations between Jewish law and washing comes at the beginning of this week's Torah portion, which commands the construction of the kiyor. This was a large basin, with twelve faucets, set in the middle of the Temple courtyard. It was required that each Kohen (priest) wash his hands and feet before beginning their daily service. (They washed their feet because service in the Temple was always performed barefoot.)
Rabbeinu Bechaya explains the deeper significance behind this ritual. He notes that since the Temple was primarily a conduit to bring God's blessing into the world, the water flowing forth from the kiyor was symbolically linked to the blessing of rain falling down upon the land.
Nachmanides (12th century Spain) says this washing ceremony was a sign of respect. He explains that just as one would never stand before a king with an unclean appearance, so too the Kohanim were particularly careful to present themselves properly in the sanctuary - the palace of the King of Kings!
Nachmanides also notes the exact procedure the Kohanim used for washing: They would first place their right hand on their right leg, and washing the two simultaneously, then repeat the same procedure on the left side. He says that the hands - when extended up above the head - are the part of one's body that most closely reach the heavens, while the feet are the most lowly part of a person. This is an allusion to the idea that one must direct simultaneously all parts of one's being in the service of God.
While it is clear that Jewish washing ceremonies are essentially spiritual in nature, there is little doubt that this has fostered a healthy approach to cleanliness and hygiene in the minds of Jews throughout the generations.