What does "like a boil that can never be cured as long as it is covered up but must be opened with all its pus-flowing ugliness" mean?

Hm ... That must be from the " Birmingham Jail" letter. It's likely they are describing racism. I'll try to decipher it. Maybe they are saying racism will never be stopped if it keeps being covered and hidden. And that it will keep coming back unless we handle it properly?