Any parasite relies on survival of the animal in which it lives in order to survive itself.
It is therefore not normal for a parasite to lead to the death of the host in which it lives. Under normal circumstances parasite and host will live in a balanced harmony. The exception arises when the normal hosts resistance against the parasite, which usually keeps it's infestation under some semblance of control, is reduced allowing the parasite to gain the upper hand. Such situations arise when wild birds are short of food, following migration, accident, injury or during extended periods of inclement weather. For the falconer's bird training, chilling, accident, or other causes of stress may trigger off the same situation.