Avoiding can imply simply that you do something to make sure it won't happen. For example you can avoid colds by maintaining a good state of health, getting enough sleep, etc. Fending off usually means that there is already some kind of attack coming, and you do something to neutralize it. For example you feel the beginning of a cold, and you take some vitamins or some special tea to inhibit the cold from taking a hold of you. So if I were to choose a verb for the two sentences above I would choose the opposite verbs than the ones that were chosen by whoever wrote the sentences.