This is the VOA Special English Health Report.

Picture a huge public gathering -- a sea of people like the hajj to Mecca or an appearance by the pope. Think of the World Cup, the Olympics, a political event, a rock concert or a train station in Asia before the Lunar New Year.

When thousands or even millions of people get together, what do you suppose is the biggest health concern?

Traditionally, doctors and public health officials were most concerned about the spread of infectious diseases, like influenza. Robert Steffen, a researcher in Switzerland, says infectious diseases are still a concern. But he says injuries are a bigger threat at so-called mass gatherings.

ROBERT STEFFEN: "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."

Mr Steffen is a professor of travel medicine at the University of Zurich. He is the lead author of one of several new papers about health problems at mass gatherings published in the journal Lancet Infectious Diseases.

Professor Steffen says children and older people have the highest risk of injury or other health problems at these events. He says children are more at risk of getting crushed in stampedes, while older people are at higher risk from extreme heat.

ROBERT STEFFEN: "At the hajj, it's particularly senior adults who attend, and so they have great risk of suffering of heat stroke and of dying."

Stampedes and crushing at mass gatherings have caused an estimated seven thousand deaths over the past thirty years.

The design of an area can play a part. There may be narrow passages or other choke points that too many people try to use at once. Professor Steffen says the mood of a crowd can also play a part.

ROBERT STEFFEN: "If suddenly they get agitated, for instance, firework is being launched within a football stadium, then they get very much afraid and try to escape."

He says organizers of large gatherings need to avoid creating conditions that might lead to panic, stampedes and heat stroke. And he says they must be ready to give medical care.

So what advice does he have for people attending a large gathering? First, get any needed vaccinations before traveling. Then, stay away from any large mass of people as much as possible. Also, be careful with alcohol and drugs, which can increase the risk of injuries.

And that's the VOA Special English Health Report, available online at voaspecialenglish.com with texts, MP3s, podcasts and captioned videos. Have you ever been in a huge crowd and worried about your safety? Or did you feel fine? Tell us about your experience. Post a comment at voaspecialenglish.com. I'm Christopher Cruise.

hajj: 赴麦加朝觐的伊斯兰教徒

laceration: 撕裂，割破

stampede: （人群的）蜂拥

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（来源：VOA 编辑：陈丹妮）