It is often believed that being sterile means being pyrogen-free but this is not correct. First of all, you need to understand what pyrogen is. Pyrogen is any substance that causes fever in animals and humans when it gets into their body. A very common symptom of fever is increasing in temperature so pyrogen is sometimes seen as any substance that causes an increase in temperature in humans and animals.
Endotoxins are very common pyrogens and they are contained in the outer membrane of the cell of some Gram-negative bacteria. When these bacteria are killed by the immune system of their hosts (usually animals and humans), antibiotics or sterilization, they release dangerous endotoxins during their cell lysis.
That being said, you should easily understand the difference between being sterile and being pyrogen-free as explained here. The process of sterilization is the killing, removal or deactivation of living microorganisms from equipment, solutions or pharmaceutical vials. So any solution that is free from living microorganisms is sterile.
However, it is possible that a solution was contaminated with some Gram-negative bacteria before sterilization and it is also possible that these bacteria released some endotoxins while being killed off during the process of sterilization. Now, this is what you must understand. Sterilization only kills off living microorganisms but it does not have an effect on their endotoxins. So, a solution can be sterile but still have dangerous endotoxins.
Being sterile means being free from microorganisms and being pyrogen-free means being free from fever-causing substances. The difference can be likened to insects and their eggs. Since some insecticides can only kill insects but not their eggs, it is possible for an apartment to be free from insects but not free from their eggs.
