In the case of plague and smallpox, I think you're getting "endemic" confused with "epidemic". Endemic means it's constantly present in a population (like flu: it's not as common as in summer, but it's still there), whereas epidemic means a sudden surge in cases locally (like flu in winter).
As for bird flu not being too transmissible: the fear is that it'll become transmissible. The thing is, there are dozens of types of "flu", some specific to certain animals. Bird flu mainly infects birds, but also infects pigs. Human flu mainly infects humans, but also infects pigs.
This is where the problems start and where the squalid conditions come in: in poorer countries pig farmers often live in close contact with their animals. That means one of their pigs can pick up bird flu from local birds and human flu from the farmers. Because of how the genetics of the virus work, when a pig has bird and human flu it's possible to get a mish-mash bird/human flu produced. The human part of the virus could allow infections of humans, but the bird part could be totally unrecognised by the human immune system so we'd be totally screwed if the combination worked out a certain way. This is why they're so scared about it: if human-infectious bird flus are appearing it means there's a good chance that sooner or later one of those forms will be highly infectious and totally new to the human immune system.
One of the last "brand new" types of flu to spread to humans was Spanish flu in 1918, reckoned to have infected a billion people and killed 20-100 million in two years.
There's no point in panicking, but a new form of flu could be very, very nasty.
100% Correct, no need to panic, but the 1918 flu is the worst medical disaster ever, so flu should never be underestimated




