When vast numbers of people gather for political or sporting events, or for concerts or pilgrimages, not everyone goes home healthy. But a new analysis finds that infectious disease is not the biggest culprit.

It may be a papal mass or the Hajj in Mecca, the World Cup, a rock festival, or a campaign rally. Mass gatherings like these often attract hundreds of thousands, even millions of people.

Traditionally, doctors and health authorities were concerned about the risk of infectious disease, like influenza, spreading through these large crowds.

That's still a concern, but no longer the biggest fear, says Robert Steffen of the University of Zurich. "The risk has actually been dominated by sprains or lacerations, or the mortality risk due to stampedes and heat exhaustion in periods of extreme heat."

Steffen is the lead author of one of several articles in The Lancet Infectious Diseases about health issues surrounding mass gatherings.

He says some age groups are at particular risk: children are more vulnerable in stampedes, for example, and older people can be at higher risk when temperatures soar.

"At the Hajj, it's particularly senior adults who attend, and so they have great risk of suffering of heat stroke and of dying," Steffen says.

Stampedes and crushings at mass gatherings have claimed 7,000 lives over the past three decades. Panic often plays a role, as does the physical layout of the facility, such as narrow passages or other choke points. The mood of the crowd can also be a factor.

"If suddenly they get agitated, for instance, fireworks being launched within the football stadium, then they get very much afraid and try to escape."

Steffen says organizers need to avoid creating conditions that might lead to panic, stampedes, and heat stroke, as well as minor injuries such as cuts and sprains. And they have to be ready to provide medical assistance.

For those attending large gatherings, Steffen suggests steering away from any large mass of people as much as possible, and getting the necessary vaccinations before traveling. And he says people should be careful with alcohol and drugs, which can increase the risk of physical injuries.