How do pathogens challenge an organism’s stability?

Pathogens are any form of a disease causing bacteria, virus or any other microorganism. When they infect an organism, that organism now has to allocate precious resources in the form of energy in order to fend off the infectious microorganisms. In some other cases, an infection could be fatal to an organism completely and "stability" wouldn't really be the biggest worry for that organism. 

In conclusion, if you're talking about a larger, more evolved organism with a working immune system. Pathogens require a lot of energy to fight, and for some small organism without a developed immune system, pathogens could throw them off completely.