Spiritual Uncleanliness
This article will discuss some interesting theology around uncleanliness in Judaism and the Bible.
The Judeo-Christian concept of uncleanliness is mutually physical and spiritual. Being spiritually unclean is the same as being sinful and the term unclean itself may also be translated as sin.
<a href="http://biblehub.com/isaiah/64-6.htm">Isaiah 64:6 is one example where the term unclean is interchangeable with the term sin. Notably this verse also states that all people are sinful. This verse is in the Old Testament so it applies within Judaism and not only Christianity.
One interesting case of uncleanliness is child birth. See the article "<a href="http://hermeneutics.stackexchange.com/questions/375/is-leviticus-12-saying-that-giving-birth-to-a-child-is-a-sin">Is Leviticus 12 saying that giving birth to a child is a sin?" When a woman has a child she is considered unclean. The modern Jewish explanation is based on this excerpt of the Talmud: "When she kneels in bearing she swears impetuously that she will have no intercourse with her husband. The Torah, therefore, ordained that she should bring a sacrifice."
Of course this is simply not true. Not all women swear that the won't have intercourse during childbirth.
Notably, modern Judaism is based on the Tanakh and the <a href="https://en.wikipedia.org/wiki/Talmud">Talmud. The earliest part of the Talmud was written around the year 200, meaning that Jesus would not have read any of it. This is not to say that he would disagree, but it is to say that a Christian cannot rely on the views of modern Jews to represent the views of Jews in Jesus' day.
One possible theory from the Christian perspective is offered in the article: "When human beings are born, this spiritual death is transmitted." According to Jewish law then, contact with a dead person renders a person unclean. So the idea is all babies are spiritually stillborn as it were, and the mother becomes unclean from contact. I'm not sure this is true but I think it is more plausible than the talmudic explanation.
In the Hebrew Bible, which is also called the Old Testament or the Tanakh, various forms of uncleanliness have different treatments or cleaning rituals. Coming into contact with a dead human corpse is considered more dire than coming into contact with a dead animal corpse, for example, and they have different purifying rituals.
Coming into contact with a dead person is called the impurity of death and it is considered by Jews to be the ultimate impurity. The purification ritual for this impurity requires a red heifer and occurs at the Temple of Jerusalem. Unfortunately, "The existence of a <a href="https://en.wikipedia.org/w/index.php?title=Red_heifer&oldid=650480621#Jewish_tradition">red heifer that conforms with all of the rigid requirements imposed by halakha is a biological anomaly." This <a href="https://en.wikipedia.org/w/index.php?title=Ritual_purification&oldid=650348164#Judaism">has lead many Jews to declare the law inactive, although all Jews "are currently assumed to possess the impurity of death."
Let's be intellectually honest here. Jewish priests don't get to declare a law of God inactive because it is too difficult for them to fulfill. The reality is that the nullification of God's law is a theological attempt to avoid the plain fact that Jews have a problem they can't solve right now, which is that they all have an impurity they cannot remove.
Some Jews are trying to fix their problem, and I really like these people because of their intellectual honesty and because of the potential that they may be helping Christians. <a href="https://en.wikipedia.org/wiki/The_Temple_Institute">The Temple Institute is a Jewish organization dedicated to establishing the Third Temple. This would be a prophetic act in both Judaism and Christianity, but the restoration of the temple involves the red heifer because no one impure can enter the temple.
<a href="https://www.youtube.com/watch?v=byAoFbA6cr8">One crazy thing is they think they found one! This cow may have been bread with the help of a <a href="https://en.wikipedia.org/w/index.php?title=Red_heifer&oldid=650480621#Christians">Christian such as Clyde Lott. Clyde is one of many Christians that believes that the Second Coming of Jesus Christ will not occur until the Third Temple is constructed in Jerusalem.