There has been many attacks/accidents involving diseases that have been purposefully used with a sole purpose to kill people. In history there has been many examples of mass deaths caused by an incurable bacteria,virus or fungi. Black death or the plague is one of them. Could the estimated 75-100 million deaths have been caused purposefully/accidentally by an experiment that ended very badly.
Yes indeed. As early as 1346, corpses with symptoms of the Black Death seem to have been catapulted by the Mongols into the city of Caffa, Crimea during a siege.
Reference: Wheelis M., Biological warfare at the 1346 siege of Caffa. Emerg Infect Dis, Sep 2002. Available here
It may be noted that this use was not very effective, since knowledge of the precise nature and transmission mechanism of the disease was not available until the 1890s. So, this is perhaps more an opportunistic use of sick individuals in a war scenario than a truly weaponised disease.