It takes only "one"
Why is it easier to notice a small blot of ink on a clean paper?
Why does it only take one matchstick to burn down a house?
Why does it only take one mistake to break someone's trust?
Why does it only take one virus to kill a man?
Why is it harder the other way around?
... to remove the ink
... to build a house
... to earn trust
... to cure illness
It's the reality but ironic