Guard Your Tongue
The severity of the צרעת, leprosy, is because it is associated with the sin of לשון הרע. There are other possible transgressions that can cause leprosy, but improper speech is the main one.
There is a seven day process of atonement, once the individual is ruled cure by the Kohein. Two birds are taken as sacrifices. One is slaughtered, and the other bird has its feet dipped in blood, and is then sent away. If that bird that is sent away, returns, it is a sign that the healed leper has not done a sincere repentance.
It seems that the theme of Parahat מצורע is the emphasis on watching what we say. Negative words can destroy a person, and take away all his dreams and hopes. The power of evil speech is very great, and its damage is immeasurable.
This is also the reason for quarantine for the leper. He created divisiveness with his speech, so he is divided in quarantine until he is cured. He can remain in that state for several, weeks, months, or years.
Clearly, it is incumbent upon all of us, to guard our tongues. This is a Mitzva that needs constant attention, and also one where there is room for improvement for all of us.