Some pathogens produce pyrogens and some are produces by the immune system.

Pathogen load is not a major factor per se in many of these. There are always exceptions. Some viruses produce high fevers and some bacterial infections low or none, but as a general rule viruses don’t really activate the arm that produces high fevers they same way and extra cellular invaders. Also as a general rule, it doesnt follow the life cycle of the organism, Also with some notable exceptions, malaria being one of them.