When it comes to the study of sexually transmitted diseases, good data is hard to come by. Most is obtained either from interviews with a small number of people or by larger random surveys. Both these methods have important limitations. The small scale surveys do not spot large scale patterns of behaviour and the larger surveys ask for the number of partners but do not record the pattern of connections between them.
Consequently, the networks of links between sexual partners are poorly understood. Earlier this year, we looked at some interesting work by Luis Rocha at Umea University in Sweden and a couple of buddies who studied a public website in Brazil in which men have graded and categorised their sexual encounters with female escorts over a period of six years. The researchers then used this data to reconstruct the network of links between prostitutes and their clients, both in time and in space.
Today, they take this work further by asking how well a sexually transmitted disease might spread through such a network. The answer is surprising. Rocha and co study the spread of HIV in particular. They point out that a key factor in the spread of the disease is the viral load.
During the chronic infection stage, the load is low enough that it requires several contacts between two individuals over a short period of time for the infection to spread. This kind of contact is rare in the network that Rocha and co have uncovered. But what of the acute stage of the disease when the viral load is greater and the chances of infection much higher? Again Rocha and co say the disease cannot spread through the network, even if the infection rate is per cent.
The reason is the time between sexual contacts. Rocha and co say there is an epidemic threshold in which the disease spreads if it is infectious for more than about a month. However, the acute stage of HIV appears to last for only for a couple of weeks. As Rocha and co point out: networks with power-law degree distributions do not generally have epidemic thresholds.
