Toxoplasma gondii
One of my long-time favorite parasites - global chances are 50/50 that you're infected. Well, depending on who you talk to, the percentages range from 30% up to 70% or more, less if you live in the USA. It's fun to watch it get broader recognition, as in the linked ABC News article. Definitely one of the most interesting (in terms of effect on humans) and widely spread parasites I've come across.
But don't forget some of the ones I linked to awhile back. That was a good Top 7 list.
Still a toss-up whether Toxoplasma gondii or the Candiru fish is scarier. Probably the fish if you're a guy, but it's fairly easy to not pee in an Amazonian river.
(Caution - don't read the Candiru article if you're squeamish - and ESPECIALLY do not check out the pictures at this site. Or the BBC video here.)