Mass gatherings, such as the London 2012 Olympics, can be a hotbed of diseases from across the world, public health experts have warned.
They say it can have consequences for the host nation and for people when they return to their own countries.
There are also important issues to consider in handling large numbers of people, they say.
A series of reports, in The Lancet Infectious Disease journal , has been highlighting the risks.
The theory is that so many people, packed closely together, increases the risk of diseases spreading.
Prof Ibrahim Abubakar, from the University of East Anglia, writes that there are risks from diseases already in the host country and from the home countries of the visitors.
He highlighted religious or music festivals and major sporting events as mass gatherings which could have a public health risk, such as an influenza outbreak during World Youth Day in 2008 in Australia.
One report said increased air travel and the spread of diseases could have "potentially serious implications to health, security, and economic activity worldwide".
The reports also highlight the challenges of managing large numbers of people and pointed to the stampede at the 2010 Love Parade in Germany in which 21 people died and 500 were injured.
Saudi Arabia has to make careful preparations for the world's largest annual mass gathering - the Hajj, with more than two million pilgrims.
Prof Ziad Memish, from the country's Ministry of Health, said: "Conventional concepts of disease and crowd control do not adequately address the complexity of mass gatherings.
"Mass gatherings have been associated with death and destruction - catastrophic stampedes, collapse of venues, crowd violence and damage to political and commercial infrastructure."
Prof Brian McCloskey, who is in charge of the Health Protection Agency's preparations for London 2012, told the BBC: "The history of the Olympic Games suggests infection doesn't happen often.
"The issue for us is to make sure the right system is in place to respond."
He has been improving the agency's disease surveillance to include data straight from hospitals and walk-in centres, which he said would "leave a legacy of probably the most comprehensive disease surveillance system in the world".