Not necessarily. A parasite could be an engineered bio-organism developed specially to infect a host, and not a natural occurrence. Now, this isn't your everyday technology.

May I remind everyone that not that many years ago there was a mass murdering of microbiologists around the globe. Hundreds died in many incidents. One can reasonably suspect it was done to reduce the number of people able to spot and sound an alarm. This was done prior to the US setting up all the overseas biowar labs it has been hiding.