Gutfernos
When the the siluro began their conquest of the land with those tentative first few steps, the gutburners were not far behind. However, as a purely aquatic genus of microbes incapable of surviving in a terrestrial environment for any extended period of time, this proved a daunting obstacle to overcome. While it was true that at this point in time it was inevitable for those land-dwelling siluro to return to the water in order to reproduce or in some lineages to keep their gills wet. Such brief exposures proved to not only serve almost too rare of an opportunity, but one which posed a great risk to them as well. Should they manage to establish all 5 stages of infection, they might potentially - albeit rarely - kill it before it could return to the water, thus killing off themselves as well. So for several thousand year the ancestors of the gutfernos remained bound to the water and only the most amphibious of siluro, until one day one lineage managed to adapt in such a way that proved such a success that its descendants would give rise to numerous lineages that still plague the terrestrial siluro to this very day.
This adaption was to become more mild, resulting in at most the worst their presence would cause was a sever case of inflammation within the gut.
Infection sites vary depending on the species. While some lurk in coastal waters where amphibious siluro dwell, others contaminate streams and still pools of water, waiting to infect terrestrial siluro which come to them to either drink or lay eggs. This is considered to be stage 1. From there, they enter stage 2 as the various gutfernos follow a similar pattern involving them infesting the lining of the gut walls. Slower reproducers than their ancestors, the infection thus spread slower as well, often giving time for the cells of the gut to replace themselves, though this does lead onto stage 3. A common symptom of this stage of the infection, besides a strong sense of thirst which lures the siluro towards water sources and thus potential vectors for future gutfernos, is the presence of blood within the stool of the host. This bloodied stool serves to help spread the gutfernos, which infest it, should it come in contact with a water source or possibly even another species of siluro. Stage 4 only occurs should the host's immune system overreact to the presence of the gutfernos, leading to inflammation of the gut walls as the body attacks itself in order to remove the foreign invaders. Stage 5 is essentially absent, as by this point in the infection the gutfernos have either reverted to stage 2 or 3, or have been flushed out of the host's gut. Because of these adaptations, the survival of the host is not at risk from their occupation of it, which in turn guarantees that they will be able to infect new hosts.
Just like in its ancestor, individual species are incredibly difficult to tell apart outside of an examination of their genome.