Flu season is upon us, but when exactly it will hit your urban jungle may depends largely on the size of the city, according to a new study published this month in the journal  Science .

Densely populated areas like cities are excellent breeding grounds for viruses. They have plenty of human hosts in which to make camp, and those hosts are spread close to other humans to whom the virus can spread via a sneeze or a cough.

Basic logic would suggest that the more people living in the city, the more opportunities for the flu to spread and the more intense an outbreak would be. Smaller cities with less dense populations would simply have a harder time getting the virus from person to person. But no one had ever actually looked at whether small or large cities experience more intense outbreaks, even though the issue is increasingly important as we try to stay one step ahead of the tricky disease.

"The increasing majority of humans now live in cities and understanding how size and structure of cities may help us predict and control epidemics," said Benjamin Dalziel, a population biologist at Oregon State University and one of the co-authors of the new paper, in a press conference.

To the researchers' surprise, however, Dalziel and his colleagues found that people living in smaller cities like Nashville and Atlanta are more likely to face an intense epidemic of seasonal flu at some point in the annual flu cycle. Denser cities, like New York or Miami, see a steady stream of flu cases, but not big outbreak all at once. The question, then, was why.

Influenza transmission

They already knew humidity plays a key role in influenza transmission. The ever-evolving virus moves between human hosts best in cool, dry months when the viruses can survive for longer outside the body. The viruses sit in liquid particles in the air, say from a cough or a sneeze.

Humidity alone could not explain the patterns in the data. So, the scientists combined climate data with medical claims data for 603 US cities from 2002 to 2008. They used census data to estimate population size and density down to the level of a city block.

"The increased resolution of the data made this study possible," says Cecile Viboud, an epidemiologist with the National Institutes for Health and co-author on the new paper. Previous research looked at flu transmission primarily at the state or regional level, says Viboud. This study zoomed in.

Flu cases

Viboud and Dalziel found that when the conditions are right in smaller cities, influenza goes crazy and results in a major spike in flu cases over a period of a couple days or weeks. In bigger cities, constant contact between individuals means influenza can easily hop from person to person without a drastic shift in the weather. These cities may see more total influenza cases than smaller cities, but the cases are spread out over a much longer period of time.

Having a model like this can also help certain cities plan ahead, says Viboud. A sudden spike in cases can stress the local health care system, so this kind of information can help city officials to predict when the flu could hit them hardest, based on the structure of their city and weather patterns, and prepare in advance.