Answer: The key to the personal a is that it is typically used only with specific, known or identified human beings (or animals or other things that have been personified). In a sentence such as the one you gave, the person speaking isn't looking for a particular person, but for a member of a certain category.
Here's another example: No conozco una sola bisabuela (I don't know a single great-grandmother), but No conozco a tu bisabuela (I don't know your great-grandmother). In the first sentence, there is no reference to a specific person, so the a isn't used. But in the second sentence, the a is needed. Similarly, Necesito una secretaria (I need a secretary) needs no a. But Necesito a la secretaria (I need the secretary), which refers to a specific person, does.
As a result, some sentences can have a slightly different meaning, depending on whether the a is used. For example, we might say that el FBI busca a un hombre de 40 años, meaning that the FBI is looking for a specific 40-year old man, perhaps the one who committed a crime. If we say that el FBI busca un hombre de 40 años, it suggests the FBI is looking for a 40-year-old man in general, perhaps to fill a role in a promotional film or for some other purpose where it doesn't particularly matter which 40-year-old man it finds.
The main exception to this rule is that certain pronouns, such as alguien and nadie, always require the personal a when used as direct objects, even when they refer to no specific person. Example: No conozco a nadie. I don't know anybody.
