Brain-eating parasite causing massive leopard shark die-offs

It enters by the nostrils, wiggles upwards through olfactory pathways and plants itself in the brain. Once invaded, victims can't be treated and will most certainly die.
The parasite is called Miamiensis avidus, and it's the pathogen most likely responsible for this year's mass die-offs of several Bay Area fish species.

Mark Okihiro, senior fish pathologist at the California Department of Fish and Wildlife, has been studying the ill-fated fishes for months now, and he recently estimated over 1,000 leopard sharks and 200-500 bat rays have washed ashore in Bay Area waters. After necropsies, Okihiro discovered the creatures' brains were ravished by an unknown invader.
Okihiro believes a brain-eating protozoan pathogen is likely responsible for the mass leopard shark and bat ray die-offs in the Bay Area.
Okihiro originally attributed the deaths to a fungal pathogen, but the mystery deepened as dead fish continued to litter beaches. Many of these corpses showed almost no signs of death by fungal invasion, although earlier samples had.

Media: Pelagic Shark Research Foundation

http://www.sfgate.com/science/article/Shark-die-offs-11824169.php#photo-12847200