The next plague of biblical proportions is currently evolving, and it's name is the bird flu. Moderate estimates put the deaths at 7 million worldwide, or around 1 in 100. One person in your high school graduating class, or ten if you went to school in a major city. One of the profs at my college. Liberal arts college students: one person you share a class with, 20 people per graduating class. Ivy League and state university students: three per class, 200 per graduating class.
Avian influenza, or bird flu as it's commonly called, is related to the same flu that humans can get. It's just enough different that most of the time humans can't catch it. If we're continually surrounded by (infected) birds that increases our chances of being able to catch it, and one particular strain called H5N1 has had a 50% human mortality rate so far. BUT you can only catch it when there's lots of infected birds around you. So far.
Viruses and bacteria are continually mutating, continually undergoing micro-scale evolution, and becoming better at infecting us. Our own bodies are also continually improving our defenses, learning to fight off unfamiliar intruders, so that most of the time we're just keeping up with the invading germs. Whenever a significantly different strain emerges our bodies have a hard time fighting them off for a while, and sometimes people die in the attacks. This has happened four times in the recorded past with the flu, the WWI pandemic of 1918-1919 being the most famous time. (There may have been other older plagues associated with the flu, but we couldn't yet determine different types of germs, or even know that they existed.)
What makes bird flu H5N1 so deadly right now is that it's not common in the human population, so we don't know how to fight it off. If the viruses manage to make it into our system, cross the moat to our castle, we don't know how to combat their tactics, so we've only a 50% chance of fighting them off, even with external reinforcements (retroviral drugs such as Tamiflu). We're lucky so far that they haven't built boats or ladders to get over the water or into our bodies and infect us more easily. Once they learn that, once they mutate, change, adapt, evolve and are able to transmit into humans more easily, and from human to human, we're gonners.
We don't have enough retroviral drugs to administer to a tenth of the population, and just because we know a tenth will likely die doesn't mean we know which ones and can innoculate them. The vaccines we have (like seeing blueprints of their battle plans before they ever lay seige to our castle) will be invalid, as they're for H5N1, and once it can get into our systems it won't be H5N1 anymore. All we can do is once they take over one castle, burn the bridges around it, raze the forests, and destroy the roads. Once one person or area is infected we will have to quarrantine it. Small towns. Villages. Neighborhoods, cities, states, countries. Even doctor's waiting rooms will have quarrantine sections so that those with coughs sit on one side, and those without on the other side. If your friends or neighbors get sick, you won't be able to drive them to the hospital, as that will mean your certain death as well. If you have the bird flu, if you can't transport yourself to a hospital, it may be better to die alone than to call for help, as you'd likely be dooming your rescuers to die with you.
Okay, I'm being more morbid than is probably really necessary. But the bird flu is scary shit. Keep an eye on it.