Asbestos is kind of green in its own right. Anthrax does come from corpses. Thus, if it's an old house, there might be a dead animal or person in it, who was once infested with b. Anthracis. Old grave robbers used to catch it from time to time rooting around in... well, graves.
It can happen. Things that die of the bacteria, or at least partially have the bacteria, will allow it to fester to the point that it uses up all it can and then reverts to its dormant 'powder' state. This can happen in the walls of a building if a rat happens to catch it and dies inside. There's more than just big ugly spiders in old houses. :)
I think Duolingo must be more strict and not forgiving these typos. I've had cases where i put the noun in the incorrect form (like forgetting the extra t) and it was considered as a typo and therefore it accepted my answer. It would be better if Duolingo would not forgive this, forces people also to doublecheck more. Same goes for when I need to put a sentence in the correct order. You can always see what the first word needs to be, since it starts with a capital character. Would be better if all words are lowercase. Just my two cents.