In talking to somebody about this tonight, there are plenty of parallel constructions in English to the heebie-jeebies discussed below, where you have or give somebody the shivers, the giggles, etc.
So, you should be able to talk about a single heebie-jeebie. But some don't work that way for me -- the jitters, where a jitter works in a technical sense, but not in this sense. The creeps is only plural as far as I can figure, though Merriam-Webster says "usually in the plural".
So, now, I just gotta know: Is there a willy behind the willies? M-W lists it as 1896, origin unknown.