Posted on 03/27/2003 7:09:34 PM PST by Lessismore
This time last month, no one had heard of Severe Acute Respiratory Syndrome (Sars) - at least because the World Health Organisation had yet to give a name to this lethal new variant of pneumonia.
This time last week, Sars had acquired its name, but no one in Britain had contracted the disease, which was still confined to faraway countries. This time yesterday the first British case of Sars had been confirmed, and now we have all heard about it.
The rapid spread of Sars is an impressive, if disturbing, demonstration of one of the most powerful natural phenomena to be identified in the past 100 years: the Small World Effect, by which just a few random links between people can turn our sprawling planet into an all-too-small global village.
The existence of the Small World Effect was first highlighted in 1967 by Stanley Milgram, a young sociology professor at Harvard University. Milgram wanted to discover the typical size of social networks - that is, how many people we count as friends, friends of friends and so on.
To find out, he posted packages to 100 people in Nebraska and Kansas, asking them to post them on to a "target" person in Massachusetts. The twist was that there were no precise details: recipients were told only his name, occupation and a few other personal details - and were asked simply to post the packet to any friends they thought might know the recipient.
Amazingly, the packets typically reached the target after just five re-postings. On the face of it, this suggests that those taking part knew about 50 others well enough to post the letter on: the first posting covering 50 people, the next encompassing the 50 friends each of these 50 had, or 2,500 people and so on, until after five postings the whole population of the United States was covered.
The problem with this reasoning is that it assumes each person's social network consists of people spread randomly across America, a patently absurd notion. Yet how else can one explain the astonishingly low number of re-postings required?
The answer emerged in 1997, during computer experiments carried out by Duncan Watts and Steve Strogatz at Cornell University in New York.
They created giant "societies" of 1,000 artificial people whose links to one another could be varied at will. As expected, Watts and Strogatz found that if each person had links only with his nearest neighbours, it took many links to get from one person to any other, while societies consisting of people with entirely random links were connected up far more effectively.
The big surprise came when the researchers took a "society" comprised entirely of short nearest-neighbour links, and replaced just one per cent of them with a random link. The "short-circuiting" effect of such links was astonishing, reducing by a factor of 10 the number of links needed to connect any two people.
The implications for the spread of diseases such as Sars are only too obvious: we need only a few random links to faraway places to end up effectively living next door to them. This Small World Effect has been amply confirmed in the past few days.
Cases in Europe, including Britain, have emerged among airline passengers, who via essentially random routes have been exposed to the pathogen responsible for Sars and then brought it back home.
Small World Theory can do more than explain the astonishing speed with which diseases such as Sars spread, however. It also points to ways of stopping them turning into a full-blown epidemic.
The conventional method is to try to protect everyone, in the hope that the disease will never acquire the critical mass of patients needed to sustain an epidemic.
Small World Theory, in contrast, suggests that we should alter the "architecture" of the links between infected and healthy people. This involves focusing resources on the relatively small number of highly "connected" people who pose a threat to a relatively large numbers of people.
The importance of these people to the spread of disease was highlighted during the early days of the Aids epidemic when investigators found that at least 40 of the first 248 men diagnosed with the disease were linked via sexual activity with one Canadian male flight attendant, whose promiscuity and profession made him, according to Small World Theory, a particularly effective initiator of an epidemic.
The profile of the people most likely to do the same for Sars is as yet unclear. Even so, if on my next long-haul flight I find myself sitting next to a consultant in respiratory medicine with a nasty cough, I think I'll ask to be moved.
Disclaimer: Opinions posted on Free Republic are those of the individual posters and do not necessarily represent the opinion of Free Republic or its management. All materials posted herein are protected by copyright law and the exemption for fair use of copyrighted works.