In 13:10-17, the passukk says if a nega tzoraas is brought to the kohein, and he sees it and it has raw flesh, the tzoraas is old in the flesh, the kohein (declares it) impure, and iof it flourishes and covers the skin, etc etc etc..
Rashi says "tzoraas is old" means it is an old wound that looks healed but underneath it is full of pus, you should not say because on top it is healing so it is tahor, but it is still tamei..
If you get into an argument, for example, with someone (your wife, husband, friend, etc.), and you let it slide. You let it go, maybe 5 years, 10 years 20 years, a week, whatever. So you have built over it many new experiences and relationships. You might think that because on top everything looks good and healthy, so everythign is fine.
but that is not true. Until the original wound is actually dealt with and repaired, it is beneath the surface festering,. You can only ignore it for so long.
The tzoraas is tamei despite the signs of healing above, because the original wound was never dealt with.
If you fight with your spouse and move on without repairing that wound, you will always bear some ill will. Even if it looks like everythign is healed, that is only on the surface, but down below the wound is festering. You have to get rid of it at the source and nip it in the bud.