It has happened once for sure and possibly three times in history. The
black death which swept China, India and Europe in the 13th century was
airborne plague bacillus. It is estimated that 20-30% of the population was
killed. Some estimates give death rates among infected individuals as high
as 90%. This rate of mortality is every bit as high as Lassa or Ebola.
When the europeans first encountered the american natives, european
diseases-small pox etc. decimated the New World populations. Some
historians ascribe population loss at 60-80% from these diseases. The flu
pandemic of 1918 took a terrible toll, it is estimated that ~18-20,000,000
people died. In a world where poverty thrives and urban centers develop
without sanitary considerations, it's only a matter of time.