At this point in the news cycle, it may be prudent to define "flu epidemic" and its far scarier sibling, "flu pandemic." "Epidemic" means simply that a sudden outbreak of the virus is spreading rapidly and affecting many people at the same time. In the UK, the National Health Services define it as when more than 400 people per 100,000 consult their doctor or go to the hospital with the flu or flu-like symptoms each week. In the US, a "flu epidemic" is defined by a percentage of deaths due to the flu or pneumonia each week. Flu epidemics happen all the time, virtually without anyone noticing: Each year on average, between 250,000 and 500,000 people die from the flu, according to the World Health Organization; in the US, an estimated 36,000 people die annually from flu-related illness.
But a flu epidemic does not mean that Pestilence, everyone's favorite Horseman of the Apocalypse, has ridden into town on his white horse and is busily cutting down healthy humans like so much grass.
Many of those flu epidemic cases can be mild, especially if caught early enough to treat with an antiviral like Tamiflu, or are lethal predominantly to the elderly, very young children, or individuals with already compromised immune systems.
A "flu pandemic," however, does mean that Pestilence has moved in and is setting up shop. A flu pandemic has two main characteristics: That it's a new strain of the virus, meaning that few people, if any, have resistance to it, and that it's managed to work its way to more than one continent.
For those reasons, a flu pandemic can be extremely deadly. The World Health Organization has defined six stages of progression leading to a pandemic flu: Phases 1 through 3 see largely animal infection, with minimal human sickness; Phase 4 is sustained human infection; Phase 5 is human-to-human contact in at least two regions; and Phase 6 is pandemic, with widespread human infection. Right now, with this current bout of swine flu,
we're at Phase 3, where the flu is causing sporadic outbreaks in limited areas "“ meaning things aren't too bad. we're at Phase 4.
"Limited transmission under such restricted circumstances does not indicate that the virus has gained the level of transmissibility among humans necessary to cause a pandemic," says WHO. Moreover, we've been at Phase 3 before, in the recent past, and Flu pandemics have actually occurred about three times every century since the 1500s. But with globalization having exploded in the years since the last one, a flu pandemic now has to the potential to kill 2 to 7.4 million people worldwide, according to WHO.
(If you want to take your mind off that scary sentence, let's talk etymology. The word "epidemic" was first used by the poet Homer, the Greek preposition "epi," meaning "on," married to "demos," the noun for "people," meaning something like residence, or living in one's country. It later took on its medical meaning after Hippocrates employed the word as the title of one of his medical treatises. For awhile, epidemic was a handle given to any collection of symptoms, from diarrhea to fevers, that affected a single area over a discrete period of time, but after the Middle Ages and the epidemics of the plague, it came to mean the outbreak a single, defined disease in an area.)