So what are we in for with this whole bird flu thing? Well first let’s understand a little about this avian influenza, and specifically the one making all the headlines right now, H5N1. The H’s and the N’s refer to where certain proteins are (the Spanish flu, discussed below, was caused by H1N1); effect on you? None. Just a name. So anyway this type of influenza virus (virus, not a bacteria) is carried by fowl. And it is infecting humans in some cases. In this limited sense, also not a big deal for most people of the world. So what’s the big deal? Right now people are only getting this stuff from birds so, well, stay away from birds and you’re cool. And that would be that if it weren’t for the ability of influenza viruses to undergo antigenic shifts, in which their characteristics can drastically change (the surface antigens are the H’s and N’s from above). So if (when) this happens, and the virus can spread from human to human, it’s like bullfrogs in Australia: the human immune system has no prior history with the invader so it doesn’t know what to do.
So when’s the last time this happened? There was the Asian Flu (H2N2) of 1957-1958 (between one and four million casualties), and related to that was the Hong Kong Flu (also H2N2) of 1968-1969 (between 750,000 and two million worldwide, including the US). But the big one, the granddaddy, was La Grippe, the Spanish Flu (H1N1), of 1918-1919. La Grippe killed between 25 and 50 million people worldwide. It started in Kansas but quickly shifted again to a more deadly version and spread to Brest, France, Boston, Massachusetts, and Freetown, Sierra Leone. It was commonly called the Spanish Flu partly because Spain had one of the worst initial outbreaks (around eight million dead) but probably more because Spain wasn’t involved in the other big news of the day, World War I. La Grippe killed more American soldiers during WWI than died in combat.

The strain was unusual in commonly killing many young and healthy victims, as opposed to more common influenzas which caused the bulk of their mortality among newborns and the old and infirm. People without symptoms could be struck suddenly and be rendered too feeble to walk within hours; many would die the next day. Symptoms included a blue tint to the face and coughing up blood caused by severe obstruction of the lungs. In further stages, the virus caused an uncontrollable haemorrhaging that filled the lungs, and patients would drown in their own body fluids.

But don’t worry. Maybe get a mask though.
Avian Flu: CDC, WHO, all else from the always wonderful Wiki.