Tum'ah is a word that is not easily defined. While we use the word "impurity" to translate the concept of tum'ah, modern man has very little grasp of ritual purity and impurity. Although we share the dread caused by the most feared source of impurity - death - it is death itself we fear, and not the state of ritual impurity it causes.
Death is modern man's ultimate fear. It is the ultimate defeat; it debilitates not only the victim, but also those left behind - loved ones, family and friends. And yet, in our experience, when death makes an appearance, the tum'ah that results is "healed" by a simple washing of the hands; a few cups of water and life goes on - at least in terms of tum'ah. In Temple times, however, a person who had come in contact with death could not enter the Temple - tum'ah and the sanctity of the Temple are mutually exclusive concepts. For the same reason, all kohanim, whose lives were intimately intertwined with the Temple service, were commanded to avoid unnecessary contact with the dead. Even today, kohanim attend funerals only for their most immediate relatives, and actively avoid all contact with death other then when absolutely necessary.
In short, we have little difficulty understanding the concept of tum'ah that has death at its source. And yet, despite the tragedy of death, the permanent and irreversible damage and void that it creates and the dread of its cruel finality, there is another type of tum'ah that is far worse: the tum'ah of tsara'at. Our sages explain that the malady called tsara'at, commonly translated as leprosy, is not the physical skin malady with which we are familiar, but rather a physical expression of a spiritual illness.
At first glance, death - and the tum'ah it engenders - seems to us far more serious and severe than any skin lesion; in fact, tsara'at might even seem trivial compared to death. Nonetheless, when we measure and compare the tum'ah that results from each of these causes, the conclusion is inescapable: The tum'ah caused by leprosy eclipses that caused by contact with death. The verses themselves illustrate the disparity: A person who came in contact with death could not enter the Temple, but the leper was completely removed from society. While we might argue that the "quarantine" of the leper was nothing more than a preventative step to avoid contagion - a step that is unnecessary in the case of tum'ah caused by contact with death - this argument overlooks the nature of tum'ah. The malady in question is spiritual, not physical. The leper is placed in isolation because he or she suffers from a contagious condition that is spiritual, not physical.
Tradition associates leprosy with sins of speech, such as gossip, slander and character assassination. The person guilty of these sins is considered spiritually dangerous, and the results of these sins are considered far more destructive than contact with death.
The first instance of the misuse of speech was in the Garden of Eden, and it was perpetrated by the serpent. In attempting to bring about disharmony between Adam and Eve, the serpent serves as the prototype of the gossiper who sows hatred and jealousy through the artful use of words. With its slick message and scaly skin, the serpent has become the quintessential image of the misuse of speech - and of the skin lesions that result.
As a result of the serpent's insidious words, mankind's grasp of truth was confounded, confused; they were exiled from the Garden of Eden. After partaking of the Tree of Knowledge of Good and Evil - a tree we might more aptly call the "Tree of Death" - they were banished, exiled, distanced from the Garden and the intimacy with God they had once enjoyed, the source of life itself. As God had warned, death came into the world. And so, we begin to see that the circle is complete: The serpent, and all those who misuse the power of speech, create a spiritual wound in human society - a wound whose physical manifestation may be likened to the skin of the snake; in human beings, this condition is called tsara'at, and it results from the same sin committed by the serpent in the Garden of Eden. Any person who behaves like the serpent must be banished, sent into temporary exile, to protect society from infection while allowing the sinner to be healed spiritually.
Slander, gossip and other serpent-like abuses of the gift of speech bring a type of spiritual death into the community, just as the serpent's misuse of speech brought physical death into the world. In both cases, the tum'ah necessarily results in estrangement, exile - either from the Temple or from all of society. And though in both cases a type of death occurs, Parashat Tazria teaches us that spiritual death is by far the greater loss.
For a more in-depth analysis see: http://arikahn.blogspot.com/2014/03/audio-and-essays-parashat-tazria.html