If you’re familiar with fungi, you’ll know that often by the time you can see a fungus, it’s already been there growing a while. I’ll be using mushrooms and mold as an example here.
The part of a mushroom that we see and call the mushroom is actually only the fruiting body. The “roots” of the mushroom are in the ground, decaying matter, or other substrate where you can’t see them. It grows there for a while before you even see the mushroom. The part you see is what happens when the fungus is ready to spread through spores instead of just staying and growing where it is. The fuzzy stuff that grows on food is similar. The fuzzy part lets you know it’s there and that it had hospitable conditions in which to grow, so it’s ready to continue elsewhere.
Sometimes sin is like that. It has specific conditions in which it can grow, and sometimes it grows in our lives without us noticing. Then before you know it you can see it, and you wonder how it got there. We must frequently look around in our lives and find the conditions that encourage the unwanted growth, and get rid of them. Sometimes you’ve left something un-turned and un-examined. Sometimes you’ve kept something too long without using it all. Even if it was good, it’s time for it to go now. Maybe it’s time to replace it, maybe you don’t need any more.
Sin tends to spread very easily. Whatever and wherever the sin is, work with God to clean it out before it further contaminates you and your surroundings, including people around you.