"Winter facilitates spread of influenza for whatever reason. And that seems to be what we've seen," Mounts says from Geneva.
As it did in the North in the spring and early summer, the virus spread quickly in the parts of the Southern Hemisphere where surveillance systems are strong enough to track it.
The sole exception seemed to be South Africa, where initially seasonal flu viruses predominated. Olsen says that may have been because the virus was introduced there later, after seasonal flu activity had already begun.
In the majority of cases, people had what seemed like regular flu. But in a small fraction of the infected, the virus's attack on the lungs was so severe that patients ended up in intensive-care units, fighting for their lives.
These folks were, generally speaking, decades younger than those who are hospitalized with seasonal flu. And doctors who treated them reported they were profoundly ill and enormously difficult to treat. And they stayed that way for prolonged periods, jamming ICUs.
"You talk to ... people who run intensive-care units, and they'll describe situations they've never seen before, with half of the intensive-care unit taken up by people with pandemic flu," Mounts says.
Influenza experts at the World Health Organization are focusing on the experiences of the temperate countries - New Zealand, Argentina, Australia, Chile and South Africa - to assess the pandemic virus's behaviour in its first true winter.