Like a boil that can never be cured if it is covered up but must be opened with all its pus-flowing ugliness
this is a sentence that i need figuring out. it is part of martin luther kings letter from birmingham jail. what type of sentence(allusion, metaphor etc.) is it and what does it mean?

It is a metaphor, as an Allusion is an expression designed to call something to mind without mentioning it explicitly; an indirect or passing reference. I haven't read the majority of the letter, so I'm not exactly able to tell you what it is. I believe it is a metaphor because I think he is referring to the pain of African Americans in their oppression at the time, almost over flowing with an ugly emotion. If you dont think I answered correctly, you should probably talk to about it with your teacher, they should be happy to help you. You can also message me if you need some clarification on what I think. :) 

It is more like a riddle and it could represent slavery or the spirit because martin luther king was all for black lives matter. what i mean and all i mean is that during slavery black people were whipped and there wounds never really healed.