When the Black Death swept europe it killed over 900,00, that was 3/4 of the population at the time, it was carried by the black rat and transmitted by their fleas. Most people actually died of the pneumonic plague, as it became airborne in a similar manner to flu.
As for the germ warfare, a lot of countries still hold viruses in vaults even after signing a treaty that they wouldn't, and yes they will say it is for research purposes in case of an outbreak. Truth is if there was a pandemic no government could handle it, I think I read somewhere that America has only 7 vaccines against sma