Okay, I'm currently driving myself bonkers trying to remember the title and author of this specific horror short-story, and to find out what anthology it appeared in. I had originally thought the story was in Under The Fang or perhaps Cabal by Clive Barker, but after checking both books, I've found its not in either one of them. *_*
Anyway, the story starts off with a married woman taking a vacation through Brazil without her family; and she is traveling by flatboat (or, otherwise, some type low-rent yacht) down the Amazon River with a man she had picked up upon arriving in Brazil. She and this man spend the day on the flatboat making love, as she is to return home to the US the following day. After an afternoon spent in bed, at dusk, the woman and her lover step out onto the deck of the boat, and she jumps because she thinks she sees reflective eyes in the jungle foliage. The woman's lover tells her a tale of shapeshifters living in the jungle; shapeshifters who at times come out and mate with mortals. She writes the story off as an interesting fable, then they go back down below deck.
When the woman goes back to the United States, she picks up her life where she left it off...husband, hauling 3 kids to soccer practice, etc. etc. Then one day not long after arriving home, she sees there is a small bite-like mark on the side of her neck. Thinking it's a pimple, she leaves it be...but then over the next few weeks, it developes into a lump. Worried that it might be cancer, the woman eventually carts herself off to see a doctor. The doctor examines the woman's neck, checks the lump, runs a few lab tests, and comes back to tells her the lump is nothing more than a cyst and to leave it alone as long as it isn't bothering her. So, she leaves it. Then a few months down the road, the woman is out in the backyard playing baseball with her husband and kids. One of the kids throws the ball and it happens to hit her in the neck... when this happens, the "cyst" bursts open...and all these baby spiders come scurrying out of her neck. 0_0 *faints*
Anyway, the story ends with the woman collapsing to theground screaming herself into madness, and the last thought she has before passing out is of her Brazillian lover telling her the story of how the shapeshifters sometimes come out of the jungle to mate with mortals.
For the life of me, I can't remember who wrote that story, but I desperately want to read it again!