@Allexio Because the creator of the poison is a sadistic bitch that enjoys fireworks, probably. Also, with a 3 day grace period before death and the blood of the exploded infecting more people instantly this poison forces people to act quickly without thinking. Probably more importantly, however, is that there likely aren't any symptoms for those who are infected, and if the masses don't know that the blood of the infected spreads the poison then they won't know that they themselves are poisoned if they happen to be nearby when someone pops. This increases the infection rate while minimizing the ability to point out those who are already infected. As such, to normal people who aren't our MC or those involved with making the poison it then becomes nigh impossible to stop the spread of the poison and save those still alive.