Plague can be treated with antibiotics now.
We have supplies of smallpox vaccine ready to go in the event of an attack.
I don’t think we actually have the Spanish flu in a lab somewhere in reproducible form. Supposedly H1N1/swine flu is descended from it, but most people have protection against severe disease now from vaccination or a previous H1N1 infection.
If someone releases a new SARS-CoV-3 that’s been engineered to be as good a killer as MERS or SARS1 while retaining the transmission characteristics of covid-19 we could be in trouble, same if someone did it with a different virus family we don’t have some immunity to already.
Bird flu could be a good example here. It is an excellent human-killer but luckily doesn’t transmit well.