I have 4 dogs. All of them are indoor/outdoor animals. 3 of them don't seem to have a problem with fleas. 1 has a big problem with them. The 1 with the big problem is very small, about 11 pounds, with hair about 1.5 inches long. I give him Frontline Tritak every month on the 1st , and Comfortis every month on the 15th. One medication alone is not enough, and the 2 combined still is not enough. I have to bathe the poor dog on the 8th and the 22nd. 2 different medications every month and a bath every other week, and the poor dog still has fleas within a couple days of each treatment/bath. He is not suffering from an allergy. I can see the fleas on him. The other 3 dogs only get the Frontline Tritak once a month, and an occasional bath, and they don't seem to have any fleas. I don't see fleas anywhere other than the small dog. So, in answer to your question, YES, some animals are more susceptible to fleas than others.