Myxobolus cerebralis
This is a problem in fish farming and also in natural fish populations. In the last 100 years, it has spread to most of Europe (including Russia), the United States, South Africa, and other countries.  In the 1980s, people found out that M. cerebralis needs to live in a worm before it can live in a fish.  It infects both the fish and the worm by poking a hole in them and putting part of itself in.