There is a new outbreak of a bird flu in eastern China, referred to as H7N9. The first thing you need to know is that human populations have not been previously exposed (to any degree) to H7 or N9 type virus, so if this virus were to mutate in such as way as to spread human to human, the result could be very serious. Moreover, the “H” component of the virus is thought to have a genetic sequence that is known to readily mutate into form that would be target (bind to) human rather than bird cells.
The virus has been found in chickens, pigeons, and ducks in markets where live birds are sold. It is not known if there is a particular species harboring the virus, acting as a reservoir, but chances are there is one species of bird out there somewhere that has been passing the virus to other birds probably in the context of the live bird markets. It is possible that the reservoir is a domestic bird or a wild bird.
Health officials are concerned because the number of human cases seems to be rising dramatically. A few weeks ago a few people were infected and there had been a few deaths. The most recent information, probably a few days old, indicates that 24 humans have been infected of whom 8 have died, across 11 cities separated by hundreds of kilometers, but with most of the cases in a couple of locations including the vicinity of Shanghai.
Also, birds with the virus appear healthy. This is different than the more famous H5N1 bird flu, which made birds sick and thus made it possible to survey for areas where the virus was around.
The more pessimistic interpretation of all of this suggests that this is a “perfect storm” of factors to start a real pandemic. The virus is spread over a large area in heavily populated areas. The most direct ways of identifying potential reservoirs is unavailable. Humans are not predisposed with any sort of immunity to the virus. Genetically, the parts that would make it a human virus are primed to do so. The mortality rate is very high. And so on.
The more optimistic interpretation would note that a fast spreading disease like this is more likely to wipe itself out quickly. A high mortality rate is often seen in the early days of an emerging disease. People got sick and died because no one was expecting it, but with a better planned response moving forward, that rate should drop. We know a lot more about flu virus genetics and transmission than even just a few years go, so the science is a powerful tool. Most importantly, there is no human to human transmission of the virus at this time. Unless that happens, this is an important public health crisis and will have impacts on the economy linked to birds, but we are not all going to die. From this. And so on.