Smallpox was released by mistake, but fortunately did not make headway because it was not a robust microbe. When they release H5N1 recombinant by mistake, or deliberately, it will not be so benign. The world's human population could have the biggest and quickest reduction ever, both proportionately and by an order of magnitude in absolute terms. Even if direct infections don't kill a billion people, the economic turmoil likely will because most people won't be going to work for fear of infection.
All the socialist calls for mass public transport will not be so popular.
It would not be beyond some Greenie true believer to think, "Hmmm, if we reduce the human population from 6 billion and rising to 2 billion and falling, that would be a great achievement. It would save the planet from CO2 so would in fact be a kindness, not just to the remaining people but to all of life, which is on a path to doom."
A deliberate release is quite possible. Islamic Jihad would think it a spiffing idea too - a huge suicide bomb.