FLU / COLD / STREP THROAT

There are probably a lot of possible logical answers to this, but here's what strikes me odd:

We don't just say "flu," we say "the flu," with the definite article.
With "cold," it's the indefinite article, as in "He has a cold."
But in the other case, it's simply "strep throat."

I guess in this case, the question wasn't
"What do these three have in common?", but
"What do these three have in uncommon?", or maybe
"What do these three have out of common?"

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