very nasty parasites can survive and enter the body from improperly handled sushi fish ... essentially, fish intended for sushi needs to be frozen at -4°F for seven days or -31°F for 15 hours to kill parasites.
article states RFKJ got mercury poisoning from eating too much large deep sea fish, so very likely the parasite came from one of those fish as well ...
On the other hand, according to the infectious disease experts and neurosurgeons citied in the article it was likely a pork tapeworm larva. The symptoms of memory loss and foggy thinking is more consistent with mercury poisoning.