Biological weapons of mass destruction have to be the worst. Unlike other weapons which would warn the enemy of the danger coming, biological weapon are the easiest to deploy and you may not even know there's been an attack until you see people dying and even then it might be impossible to tell who is responsible for the attack as it's possible for just one infected person to spread a disease around, get hundreds of thousands of people infected.
Yeah, actually most of the biological warfare in the past had used existing viruses and germs. It is even hard to tell if it was indeed a biological warfare or a natural occurring disaster until you can get some evidence.
Japanese Unit 731 had air dropped some black death type of rat viruses on small scales in China during WWII. Other than that, I am not sure if there was any real biological warfare taking place.
Think about how many viruses and germs naturally exist in the world. Many of them are still incurable. Many more of them aren't even discovered yet.
I remember watching a documentary about some scientists randomly caught a few mosquito in the jungle, and detected more than 20 previously unknown viruses.
So all the existing viruses can be used as weapons. Not to mention if someone make them more deadly by combine different viruses.
Another documentary I watched in the past had said that if someone combine rabies and flu viruses, then we will have a "28 days later" type of zombie apocalypse. It is scary to even think about it.