Why use The instead of A? Perhaps the writers of The are pulling at our emotions.
closed as not constructive by tchrist, aedia λ, cornbread ninja 麵包忍者, simchona♦ Feb 12 at 22:22
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Calling something the silent killer does not necessarily mean it is the exclusive silent killer, simply that it is the one of central interest in a particular context. If I am borrowing someone's computer and ask "what is the password?" to log on, I am asking for a password that will work for me, regardless of the fact that there may be dozens of active accounts and thus dozens of working passwords on that computer.
Advocates creating a public service advertising campaign in order to raise the profile of a particular topic would always want to use the instead of a. Calling carbon monoxide, or hypertension, or chronic inflammation the silent killer suggests that it is, if not the only silent killer, perhaps the most important among them (at least until the next PSA comes on). A silent killer is just one of a bunch— and not necessarily the worst of the bunch.
Nor is the habit limited to health activists. Why, just in carbonated soft drink slogans alone, off the top of my head, I recall
which all imply a uniqueness to the flavored sugar water contained in a can bearing certain logos, as opposed to that in cans with other logos.