El Elvis Rojo and everybody else who wants the book spoiled: Here ya go.
The way King explained the aliens in the book, they were an orange fungus or spore who appeared to be big-headed little gray men because the people that saw the aliens expected them to look that way. The way I understood it, the aliens were simply spores with a hive-mind. Inhaling or touching the spores gave you a funky rash as the spores attached to your body and started to grow, like a fungus. Ingesting the spores gave you a case of the shit-weasels, which explode out of the victim’s nether regions and begin to lay eggs containing more shit-weasels. Someone with a superior ability to suspend disbelief than I possess can try to explain the relationship between the spores and the shit-weasels; I still don’t clearly understand it. The idea is, I think, to infect the human race with shit-weasels, who are telepathically controlled by the spore-people, and thus gain total control of mankind. Apparently, the spores had colonized other planets the same way. Unfortunately for the fungus-folk, Earth’s climate is extremely hostile to their survival (they landed in the Maine woods in the dead of winter, fer cryin’ out loud) and the human body is not an ideal host for the shit-weasels. Instead of staying small and unobtrusive in the host’s intestines like they’re supposed to, they grow very large very quickly and kill their hosts instead of taking control of them. Except for one man. (cue triumphant crescendo) A spore successfully implants itself into our hero, takes the last remaining shit-weasel which is growing inside a collie, and heads towards Derry to dump the dog into Boston’s water supply. Hilarity ensues when our hero’s best friend teams up with a disgraced military man and his childhood friend who has Down’s syndrome and is telepathically gifted. They chase Spore-man to Derry while being pursued themselves by a certifiably whacko Army general. If you’ve read much King, you can probably write the rest of the book yourself.
And this book is one of the many reasons why I no longer buy King in hardcover.