For a pandemic to occur, each person infected needs to, on average, infect one or more others.
And finally, every so often a work tries to use the horror of pandemic to show something good.
Forty years on from his debut Night of the Living Dead, George A Romero reintroduces a zombie pandemic to America – like they don't have enough social problems already.
When HIV-AIDS hit Uganda hard in the 1990s, women volunteers showed the wider world how to reverse the pandemic, to demystify it with counseling and community care.
The first great plague pandemic to be reliably reported occurred during the reign of the Byzantine emperor Justinian I in the 6th century ce.
Professor Sir Liam Donaldson, the government's chief medical adviser, said: "Phase five indicates that WHO considers a global pandemic to be imminent, whereas at phase four a global pandemic is not inevitable.
So while others date the start of the HIV pandemic to the 1980s, for Wolfe it began 8m years earlier when the forest-dwelling ancestor of humans and chimpanzees began to hunt and butcher monkeys.
Experts say that for a pandemic to occur, the virus would have to change in a way that makes it more easily transmissible from birds to people and from person to person.
Electronic Arts, for example, until this week the largest independent games-publisher (Activision Blizzard will be bigger), recently bought two studios, BioWare and Pandemic, to strengthen its position in role-playing and action games.
Moreover, Dr Kawaoka also found that H7N9 is less sensitive to neuraminidase inhibitors (drugs such as Tamiflu) than was the H1N1 virus that became pandemic, to much consternation, in 2009.
If the new subtype causes severe disease in humans, spreads easily between people, and has a combination of surface proteins to which few people have immunity, the stage will be set for a new influenza pandemic to occur.
