By why did one particular infection start a pandemic?

NPR: Origin of AIDS Linked to Colonial Practices in Africa

We also recommit to preventing the spread of this disease, fighting the stigma associated with infection, and ending this pandemic once and for all.

WHITEHOUSE: Presidential Proclamation -- World AIDS Day, 2012 | The White House

Last year's flu-shot shortage was scary enough, but now scientists see the specter of a bona fide pandemic if the bird-flu strain known has H5N1 were to mutate into an infection that could spread to people.

FORBES: Magazine Article