The condition in which an endemic pathogen depends on the type of pathogen, transmission mode, incubation period, environment, etc, to determine the causes can be very specific for each case, however, from the listed scenarios, we can see that a stressed population has lower defenses plus overpopulation makes easier the disease transmission, in the second case, we know that mutation is one of the great cause of pathogens success, finally if the vector of a certain disease increase suddenly its populational numbers can infect more hosts, therefore we can say that the correct answer is All of these choices are correct.