Ah. Now I see. What you call 'accident' I call 'event', because such a great thing as alternation has consequences for most areas of an organism's life. It might have gone the other way, but that it is still so widespread is exactly as accidental as the placement of a brick in the middle of the wall.

If I am not mistaken (I can look it up sometime), practically all plants and fungi have it. In lower animals, some stages of a life cycle can be mobile founders, and later ones would live in tightknit colonies. Asexual/sexual alternation is pretty common, too. See wiki on trematodes.

No idea about Ginger's aliens, except that they just might be accidentally transmitted to humans when humans eat their 'rightful' hosts. Some frog parasites go on living in snakes... (Is that what you mean?)