What is the definition of Undefined Infection?

An 'undefined infection' is like a mysterious germ that sneaks into your system and refuses to be named or treated. It's the kind of infection where your doctor gives you a prescription but can't quite pinpoint its identity, leaving you scratching your head (or rather, your skin) in confusion and possibly developing some serious rashes.

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Here's an unsettling example sentence: "The 'undefined infection' that spread through the underground bunker was so elusive it earned the moniker 'The Lost Molecule,' and by the time we finally named it, it was as elusive as a ghost in a disco ball."