A boil and a burn don’t combine, they don’t spread from one to the other, they don’t spread onto skin, and a nega on skin doesn’t spread to them. If they were still forming, they are ritually clean. If they scab over as thick as a garlic peel, this is the scar that the Torah mentions. If they later heal, even if they leave a mark, they are treated as regular skin.
Rabbi Eliezer was asked about a person who had a white spot the size of a sela (a denomination of coin) form inside his hand and it covered up the scar of a boil. He replied that such a person should be quarantined. They asked him why this should be, seeing that it can’t grow white hair and it can’t spread, plus healthy flesh wouldn’t render it unclean. Rabbi Eliezer replied that it’s still possible that the spot will contract and then spread. They then asked him, “What if it’s only the size of a split bean?” to which Rabbi Eliezer replied that he had not heard anything. Rabbi Yehuda ben Baseira then said to Rabbi Eliezer, “May I teach something on this matter?” Rabbi Eliezer replied, “If you’re going to support the words of the Sages, then yes.” Rabbi Yehuda ben Baseira then taught that such a person is quarantined because another boil might appear and spread into it. Rabbi Eliezer then said, “You are a great scholar because you have supported the words of the Sages!”