Biological Threats anyone??

for my final battle scene, i need a huge population to be in jeopardy. we're talking about at least a quarter of the inhabitants of earth, possibly more. so any ideas for biological threats that could be easily unleashed and that are hard to control that could wipe out a ton of people in a relatively short amount of time (under a year). i want to stay away from the classic reintroduction of the black plague, but something a bit more unique and along those lines would be helpful

MRSA or some other equally deadly and virulent thing. Can't go wrong with a germ, though most of them do the same thing differently.

How about this; exterminators are using this new, non-poisonous method of killing termites. They use something that can flush all the wood-digesting microbes out of the termite with severe diarrhea so that all the termites starve to death. But the method is then applied to humans and after everyone gets their bowels purged, they have difficulty digesting anything that isn't yogurt.

It's going to be very messy...

The influenza epidemic of 1918 killed 50 to 100 million people (3% to 6% of the world population at that time). It was a highly virulent H1N1 form that triggered the worst responses in the healthy young adult population. The stronger the victim's immune response, the sicker the victim becomes. [This also occurs with hanta virus, making the prime fatality usually mid-life, not very young and not very old.]
Wikipedia actually has a decent write-up on th 1918 pandemic.

H1N1 attacks the middle ground of victims the strongest and most fataly--the ones who make up the armies. LIttle works against any virus.
Then again, consider Sarin gas. Quickly deadly. Not easily treated. Kills indiscriminately.

Prions (virulent proteins which fold other proteins into copies of themselves) could easily be slipped into just about anything, and could prove very difficult to trace. Worse, once you have them, there's no treatment; "mad cow disease" (bovine spongiform encephalopathy) is the most famous prionic disease. I read a short story where a plague of prions was eventually traced to the licking adhesive on mail envelopes.

wow thanks everyone for the ideas!

This is one I'm making up, kinda. A virus that is similar to AIDS, but rather than shut down the immune system, it makes it go into hyperdrive, closing off all cells entirely. Eventually, the skin cells die, can't be replaced, and become a hard shell. The brain and other internal organs under go necrosis, turning into rotten soup, and then pour out of pores, ears, nostrals, eyes, any hole in the body. It has no cure, and will spread through air, water, food, animals, fleas, blood, anything.

Weaponized smallpox or Bubonic Plague. If you doubt the killing power of those illnesses, remember that the Bubonic Plague killed anywhere between 40 and 60 percent of Europe alone, not to mention the other areas it affected. Smallpox didn't such a concentrated killcount, but it was spread out over time. The worst thing is that we have these already. Stored in CDC centeres across the nation. If someone were to turn it into an aerosol and the containment levels failed....

Let's just say it wouldn't be a pretty sight. Sure, it would take some time, but both of those illnesses have horrifyingly painful symptoms.

Inhalation anthrax. Starts with flu-like symptoms, ends with respiratory collapse. If it's discovered early, you have a pretty decent chance of surviving, but if it's discovered after the respiratory collapse kicks in, you can kiss your life goodbye. Death occurs at around a week after exposure, which would put a massive strain on any attempts to protect the general populace.

Anthrax cannot be transmitted from human to human, but the spores can survive in the wild, and in your corpses for ages. You could burn corpses, and bury them as deeply as you can, but that will take ages and lots of manpower. In addition, people in charge of destroying the bodies would need to stay on a large course of antibiotics, which could create antibiotic-resistant strains of other diseases.

Airborne Ebola with a longer incubation and infectious period.