A disease that causes foaming of the mouth spread by bite. Once infected there's no going back, and you too have to bite people to spread the infection
A thought that circles back on itself in a never ending infinite loop like a snake eating it's own tail, but leads the thinker to epiphanies, like when Kekule discovered the shape of the Benzene ring.
The chicken or the egg which came first, contemplation of this benzenous thought led the Buddha to moksha