The black plague, aids, bird flu and other virulent strains --can we prevent another plague?

A plague is inevitable. Bacteria and viruses are constantly mutating. They are adapting for survival, taking the bits and piece from other bacteria around them that may do them good, developing antibiotic resistance, and finding new ways to infiltrate our bodies. Drug/vaccine research and development takes time, and by the time people have acquired a new disease it will most likely have spread.
It is inevitable. That being said, it may be a while before another plague actually happens. They were much more common before proper sanitation techniques were around, and now hospitals and health care systems know a lot more about how to stop the spread of disease. So, we are safer than we were in the 1700's... maybe. Don't forget we also have tons of antibiotic resistant bacteria just waiting to rear it's ugly head, and viruses are usually untreatable.
Also keep in mind that medical technology is better than it has ever been. We have a lot of resoures on our hands, and should a plague occur we will be better equipped to treat it and control it then were 300 years ago, or even 50 years ago for that matter.

you're watching too much TV
Sadly, there is a great deal of evidence to suggest that "Pandora's Box" has already been opened.
No, even if we prevent one, since viruses mutate there can be another one. The box was never closed to star with.
You're comparing apples and oranges. The "black plague" was bacterial (bubonic plague); today, it could easily be treated by antibiotics. The others are viral; in time, I feel reasonably certain that we'll have vaccines for all of them.
You ask a good question with no easy answers. Consider the fact that all three pandemics in the 20th century were bird flu's. They are notoriously difficult to eradicate and are constantly mutating. Add to that the fact that Ebola Zaire was 90% lethal, yet no common terrestrial cause was found, and a pandemic becomes increasingly difficult to predict or even stop. I saw an interesting theory that made sense, though, that there were no terriestrial causes found for Ebola Zaire possibly because at the time that Ebola Zaire appeared that an asteroid exploded over the region. So one theory is that Ebola is an extraterrestrial virus. If this is the case, and it has been shown that some viruses can live in a vacuum, then we have more things to worry about than an avian flu.
Scientific studies in microbiology.