This song is about leprosy, which is a disease that was talked about many times in the Christian Bible. It is most likely referencing the verse Leviticus 13: 3-6
"3 The priest shall look at the mark on the skin of the body, and if the hair in the infection has turned white and the infection appears to be deeper than the skin of his body, it is an infection of leprosy; when the priest has looked at him, he shall pronounce him unclean. 4 But if the bright spot is white on the skin of his body, and it does not appear to be deeper than the skin, and the hair on it has not turned white, then the priest shall isolate him who has the infection for seven days. 5 The priest shall look at him on the seventh day, and if in his eyes the infection has not changed and the infection has not spread on the skin, then the priest shall isolate him for seven more days. 6 The priest shall look at him again on the seventh day, and if the infection has faded and the mark has not spread on the skin, then the priest shall pronounce him clean; it is only a scab. And he shall wash his clothes and be clean."
At the end of the song he says "We are all like lepers.
Without Christ's love in us.
Let him heal you let him.
Heal you let him will you".
When he says 'we have leprosy' he means we all have sin, and we should ask God into our heart to 'cleanse' us of our sins.