The HIV virus responsible for AIDS cannot be transmitted via the air -- it requires fluid exchange, primarily in the form of blood or semen and even then, it requires some method of entering the body (for example through mucous membranes or open cuts, sores, etc.) to actually facilitate infection.
HIV is actually quite difficult to contract outside of unprotected sexual intercourse or certain healthcare settings when blood is present (for example during medical procedures that require incisions, tooth extractions, etc.)
Bottom line - you will not catch AIDS from merely being in the presence of an HIV positive person, nor can their breath transmit the virus.
You are correct that Pneumonic plague can be spread person-to-person via inhalation of tiny droplets of that become airborne when the person coughs. Bubonic, on the other hand, requires a vector to spread -- for example via fleas or handling infected animals. That said, both of these diseases are extremely rare. Even bubonic and pneumonic plague can be successfully treated with antibiotics, provided the individual is diagnosed quickly.
The bubonic plague killed so many in the Middle Ages because there was a fundamental lack of knowldge of hygiene and how disease could be spread via animals, as well as no antibiotics to treat people.
For some perspective, you have a better chance of dying in a car crash on your way home from the pharmacy than contracting any of these airborne pathogens.
So wear your seat belt! 
