Plague (bacillis pestis) breaks out badly in an Algerian port city. The officials go into deep deep denial at the phase when strong medical and quarantine measure interventions might have stopped it.
The "chamber of commerc" types scream to the mayor "we will be ruined if you quarantine these areas". So the plague gets a strong grip.
By the time that there is no hiding it, it is too late, and the mayor declares quarantines, but the panicked citizens flee in terror in all directions.