Smallest organism to cause amnesia (in humans)

In 1996, commercial fishermen on the Pocomoke River and estuaries close by at Chesapeake Bay in Maryland, USA began suffering from an unexpected set of maladies, including headaches, skin irritation, weight loss, and, most sinister of all, inexplicable bouts of memory loss. In some cases, the men were unable to remember their own telephone numbers, and even the purpose or destination of journeys after having set out in their cars. Fortunately, after 3-6 months they recovered, and when similar cases were reported in the North Sea in 1997, scientific investigations uncovered the memory-stealing culprit. It proved to be a species of tiny single-celled dinoflagellate, no more than 0.02 mm across, called Pfiesteria piscicida. [Ref: The Lancet, August 1998.]