These airborn diseases can be passed on not only from human to human, but from animals such as rats and birds. Once human to human contact spreads the virus across a large region, or even globally, it is considered by the World Health Organization to be phase 6 of pandemic proportions. This can result in millions - or even billions - of casualties. Such loss of life would be something not seen for generations - the last major catastrophe being the Spanish Flu pandemic of 1918-1919.
There have been other relatively small-scale pandemics since then including such disasters as the Hong Kong Flu of 1968-1969 (34,000 deaths) and the Asian Flu of 1957-1958 in the U.S. (70,000 deaths). Other notable pandemics throughout history are the Black Death of the middle ages, Measles and Smallpox.
In the future we will have to watch out for more diseases such as the Bird Flu and the Ebola Virus. There are many highly contagious viruses that must be kept in check and many more that we don't know about yet. Something as simple as the common cold could have a mutation so devestating it could become the next Spanish Flu. According to the World Health Organization, SARS has been eradicated. That sounds promising and reassuring, but do we really know where it came from to begin with? Some animal could be walking around rural China as you read this with a highly more contagious and deadly mutated form of the SARS virus. If it happens to cross paths with a few Chinese farmers on the way to the market, we could be in trouble, especially if they're carrying some Bird Flu infected chickens with them.