Holy cow yes. Sterile environments are a big deal. Try holding one for a day, let alone a work week or a year. People screw up all the time, and bacteria, being pretty much invisible, are real tough to ferret out. Let alone all the actual non-sterile stuff you want to do in one. Think a clean room with bunny suits, that is the type of environment you need just to get a start on figuring out Dr. Doom type viruses and all that awful mumbo-jumbo. It takes a lot of energy, time, and resources to just get off the ground.
I do not think you need a safe lab to produce a viable weapon grade virus.
Here is basic outline: start with existing virus that has strong desired traits and known strains that mutated to resist antibiotics. Example traits include spread model, incubation length, and lethality.
Establish or take over a remote site that has little to no interaction with outside world. Remote corners of Africa and South America come to mind, there are plenty of secret illicit drug farms in the jungle. [0]
Infect the sample population with target disease, give it a few days, and slowly start to drip in countermeasures gradually increasing the dose. Idea is similar to how diseases we get anti-biotic resistant strains in the first place - people do not complete the full course of drugs and are left with weakened, but also with a strong selective pressure that benefits against strains that have resistance against drugs person was treated with.
Take samples when you have desired output and continue with new group of people.
To account for people who are immune to a particular disease repeat this with a different disease, potentially one that can advantage of weakened immune system.
Once target disease(s) are ready distribute them in population centers.
Now there are few obvious cons I can think of:
1. If secret about this leaks out, military reaction form rest of the world would be swift.
2. Hiding something like this is hard, and get's exponentially harder as group grows.
3. There is a strong chance something like this was tried already and failed. Possibly because I am grossly underestimating immune system.
4. To keep initial phase of developing secret initial group must be small, to spread it effectively dissemination group must be large.
[0] Another potential avenue is partnership with a supportive state such as Syria, North Korea, or Iran.
Basic precautions like light protective wear[0] and heavy dose of anti-biotic. If that does not work - great our virus now can jump protective wear, anti-biotic is no help, and there are less lose ends. Of course there needs to be some kind of full hazmat extraction team team that understands virulence of what they are dealing with in order to clean up. In case of state with lose morals helping this might be easier because you could use prison as a site and have full hazmat personal safe from prying eyes.
[0] not a full hazmat, just some protective wear over mouth, nose, ears, and eyes.
